Financial Institutions (FISI) 424B5 Prospectus supplement for primary offering

Filed pursuant to Rule 424(b)(5)
Registration Statement No. 333-283475

PROSPECTUS SUPPLEMENT

(To Prospectus Dated December 4, 2024)

4,000,000 Shares

Common Stock

We are offering 4,000,000 shares of our common stock, par value $0.01 per share.

Our common stock is listed on the Nasdaq Global Select Market, or Nasdaq, under the symbol “FISI.” The last reported closing price of our common stock on Nasdaq on December 6, 2024 was $26.73 per share.

Investing in our common stock involves risks. Before investing in our common stock, you should consider the information under the heading “Risk Factors” beginning on page S-12 of this prospectus supplement, on page 5 of the accompanying prospectus and under the heading “Part I. Item IA. Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2023 which is incorporated herein by reference.

	Per Share		Total(1)
Public offering price	$	25.00	$	100,000,000
Underwriting discounts and commissions(2)	$	1.25	$	5,000,000
Proceeds, before expenses, to us	$	23.75	$	95,000,000

(1) Assumes no exercise of the underwriters’ option to purchase additional shares described below.

(2) The underwriters will also be reimbursed for certain expenses incurred in this offering. See “Underwriting” for details.

We have granted the underwriters an option, exercisable not later than 30 days after the date of this prospectus supplement, to purchase up to an additional 600,000 shares of common stock from us on the same terms and conditions set forth above.

None of the Securities and Exchange Commission, any state securities commission, the Federal Deposit Insurance Corporation, or the FDIC, the Board of Governors of the Federal Reserve System, or the Federal Reserve, the New York State Department of Financial Services, or the NYSDFS, or any other regulatory authority has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Shares of our common stock are not savings accounts, deposits or other obligations of any bank or non-bank subsidiary of Financial Institutions, Inc. and are not insured or guaranteed by the FDIC or any other governmental agency.

The underwriters are offering the shares of our common stock as set forth in the “Underwriting” section of the prospectus supplement. The underwriters expect to deliver the shares of common stock in book-entry form only through the facilities of The Depository Trust Company, against payment on or about December 13, 2024.

Sole-Bookrunner

Keefe, Bruyette & Woods

A Stifel Company

Co-Manager

Piper Sandler

Prospectus Supplement dated December 11, 2024.

TABLE OF CONTENTS

Prospectus Supplement

ABOUT THIS PROSPECTUS SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS	S-1
WHERE YOU CAN FIND MORE INFORMATION	S-3
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	S-4
SPECIAL CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS	S-5
PROSPECTUS SUPPLEMENT SUMMARY	S-8
THE OFFERING	S-10
RISK FACTORS	S-12
USE OF PROCEEDS	S-17
CAPITALIZATION	S-18
MATERIAL U.S. FEDERAL INCOME TAX CONSIDERATIONS TO NON-U.S. HOLDERS	S-19
UNDERWRITING	S-24
LEGAL MATTERS	S-29
EXPERTS	S-29

Prospectus

ABOUT THIS PROSPECTUS	1
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS	1
FINANCIAL INSTITUTIONS, INC.	3
RISK FACTORS	5
WHERE YOU CAN FIND MORE INFORMATION	5
USE OF PROCEEDS	6
THE SECURITIES WE MAY OFFER	6
DESCRIPTION OF DEBT SECURITIES	7
DESCRIPTION OF COMMON STOCK	13
DESCRIPTION OF PREFERRED STOCK	16
DESCRIPTION OF DEPOSITARY SHARES	17
DESCRIPTION OF WARRANTS	18
DESCRIPTION OF PURCHASE CONTRACTS	20
DESCRIPTION OF UNITS	21
PLAN OF DISTRIBUTION	21
LEGAL MATTERS	23
EXPERTS	23

ABOUT THIS PROSPECTUS SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS

This document is composed of two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock, including the price, the number of shares of our common stock being offered, the risks of investing in this offering of our common stock and certain other matters relating to us and our financial condition. This prospectus supplement also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into this prospectus supplement and the accompanying prospectus. The second part of this document is the accompanying prospectus, dated December 4, 2024, which is included as part of our shelf registration statement on Form S-3 (File No. 333-283475). That registration statement and the accompanying prospectus provide more general information about securities that we may offer from time to time, some of which may not apply to this offering. It is important for you to read and consider carefully all information contained or incorporated by reference in this prospectus supplement, the accompanying prospectus and any permitted free writing prospectuses we have authorized for use with respect to this offering before investing in our common stock. See “Where You Can Find More Information” and “Incorporation of Certain Documents by Reference” for additional information.

This prospectus supplement, or the information incorporated by reference in this prospectus supplement, may add, update or change information in the accompanying prospectus. If information in this prospectus supplement is inconsistent with the accompanying prospectus, this prospectus supplement will supersede the information in the accompanying prospectus.

In various places in this prospectus supplement and the accompanying prospectus, we refer you to sections of other documents for additional information by indicating the caption heading of the other sections. All cross-references in this prospectus supplement are to captions contained in this prospectus supplement and not in the accompanying prospectus, unless otherwise indicated.

In this prospectus supplement, “we,” “our,” “us,” “FISI” or “the Company” refers to Financial Institutions, Inc., a New York corporation, and our consolidated subsidiaries, including Five Star Bank, a New York chartered bank, unless the context indicates that we refer only to the parent company, Financial Institutions, Inc. In this prospectus supplement, the “Bank” refers to Five Star Bank, our banking subsidiary. Unless otherwise indicated, information presented in this prospectus supplement assumes the underwriters’ option to purchase additional shares from us is not exercised.

Neither the Company nor the underwriters have authorized anyone to provide you with any information other than that contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or in any free writing prospectus prepared by or on behalf of, the Company, or to which the Company has referred you. Neither the Company nor the underwriters take any responsibility for, or can provide any assurance as to the reliability of, any information that others may give you. If any information in this prospectus supplement is inconsistent with the accompanying prospectus or any document incorporated by reference in this prospectus supplement or the accompanying prospectus, you should rely on the information in this prospectus supplement. You should not assume that the information provided in this prospectus supplement, the accompanying prospectus or the documents incorporated by reference in this prospectus supplement and in the accompanying prospectus is accurate as of any date other than the date of this prospectus supplement or the date of the document in which that information is contained. Our business, financial condition, liquidity, results of operations and prospects may have changed since the date of any document in which such information is contained.

Neither the Company nor the underwriters are offering to sell nor seeking an offer to buy our common stock in any jurisdiction where such offers and sales are not permitted. The distribution of this prospectus supplement and the accompanying prospectus and the offering of our common stock in certain jurisdictions may be restricted by law.

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Persons outside the United States who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about and observe any restrictions relating to the offering of our common stock and the distribution of this prospectus supplement and the accompanying prospectus outside the United States. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used for or in connection with, an offer or solicitation by any person in any jurisdiction in which such offer or solicitation is not authorized or in which the person making such offer or solicitation is not authorized or is not qualified to do so or to any person to whom it is unlawful to make such offer or solicitation, and this prospectus supplement and the accompanying prospectus may not be delivered to any person to whom it is unlawful to make such offer or solicitation. See “Underwriting” in this prospectus supplement.

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WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the United States Securities and Exchange Commission, or the SEC. Our filings with the SEC are also available to the public through the SEC’s website at www.sec.gov.

Our annual, quarterly and current reports and any amendments to those reports are also available over the Internet at www.FISI-investors.com under the “Financials” page. All internet addresses provided in this prospectus supplement or the accompanying prospectus are for informational purposes only and are not intended to be hyperlinks. In addition, the information on, or accessible through, our website, or any other website described herein, is not a part of, and is not incorporated or deemed to be incorporated by reference in, this prospectus supplement or the accompanying prospectus or other offering materials.

We have filed a shelf registration statement (File No. 333-283475) with the SEC registering the offering of various of our securities, including the shares of our common stock offered by this prospectus supplement and the accompanying prospectus. This prospectus supplement and the accompanying prospectus are part of that registration statement. The registration statement may contain additional information that may be important to you. Statements contained in this prospectus supplement and the accompanying prospectus as to the contents of any contract or other document referred to, or incorporated by reference, in this prospectus supplement and the accompanying prospectus are not necessarily complete and, where that contract or document is an exhibit to the registration statement, each statement is qualified in all respects by the exhibit to which the reference relates. You may obtain from the SEC copies of the registration statement and the related exhibits that we filed with the SEC.

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INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC’s rules allow us to incorporate by reference information into this prospectus supplement and the accompanying prospectus. This means that we can disclose important information to you by referring you to another document. Any information incorporated by reference into this prospectus supplement and the accompanying prospectus is considered a part of the information contained herein and therein. We are incorporating by reference in this prospectus supplement, and have incorporated by reference in the accompanying prospectus, the documents listed below, which we have already filed with the SEC, and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, or the Exchange Act, except in each case as to any portion of any report or document that is deemed furnished to the SEC and not deemed filed under such provisions:

• our Annual Report on Form 10-K for the year ended December 31, 2023, filed with the SEC on March 13, 2024;

• the portions of our Definitive Proxy Statement on Schedule 14A, filed with the SEC on  April 12, 2024 that are incorporated by reference in our Annual Report on Form 10-K for the year ended December 31, 2023;

• our Quarterly Reports on Form 10-Q for the quarter ended March 31, 2024, filed with the SEC on May 6, 2024, for the quarter ended June 30, 2024, filed with the SEC on August 5, 2024, and for the quarter ended September  30, 2024, filed with the SEC on November 4, 2024;

• our Current Reports on Form 8-K filed with the SEC on March  14, 2024, March  27, 2024, April  1, 2024, June  6, 2024, June  25, 2024, and September 16, 2024; and

• The description of our common stock set forth in the registration statement on  Form 8-A, filed with the SEC on June 23, 1999, to register our common stock under the Exchange Act, as updated and amended from time to time.

The information contained in this prospectus supplement and the accompanying prospectus will be updated and supplemented by the information contained in the filings we make with the SEC in the future and that are incorporated by reference into this prospectus supplement and the accompanying prospectus as described above. The information contained in those future filings will be considered to be part of this prospectus supplement and the accompanying prospectus and will automatically update and supersede, as appropriate, the information contained in this prospectus supplement and the accompanying prospectus and contained in the filings previously filed with the SEC that are incorporated by reference into this prospectus supplement and the accompanying prospectus.

Upon written or oral request, we will provide, without charge, to each person to whom a copy of this prospectus supplement and the accompanying prospectus is delivered a copy of the documents incorporated by reference into this prospectus supplement and the accompanying prospectus. You may request a free copy of these filings by writing or telephoning us at the following address:

Financial Institutions, Inc.

Attn: Samuel J. Burruano Jr.

Corporate Secretary

220 Liberty Street, Warsaw, New York 14569

Telephone: (585) 786-1100

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SPECIAL CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus, and the documents incorporated by reference herein and therein include “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. These forward-looking statements represent plans, estimates, objectives, goals, guidelines, expectations, intentions, projections, and statements of our beliefs concerning future events, business plans, objectives, expected operating results, and the assumptions upon which those statements are based. Forward-looking statements include, without limitation, any statement that may predict, forecast, indicate, or imply future results, performance, or achievements, and are typically identified with words such as “may,” “could,” “should,” “will,” “would,” “believe,” “anticipate,” “estimate,” “expect,” “aim,” “intend,” “plan,” or words or phases of similar meaning. To the extent that this discussion describes prior performance, the descriptions relate only to the periods listed, which may not be indicative of our future financial outcomes. We caution that the forward-looking statements are based largely on our expectations and are subject to a number of known and unknown risks and uncertainties that are subject to change based on factors which are, in many instances, beyond our control. Such forward-looking statements are based on various assumptions (some of which may be beyond our control) and are subject to risks and uncertainties, which change over time, and other factors which could cause actual results to differ materially from those currently anticipated. Such risks and uncertainties include, but are not limited to:

• Fluctuations in market interest rates may affect our interest margins and income, demand for our products, defaults on loans, loan prepayments and the fair value of our financial instruments;

• Environmental, social and governance matters, and any related reporting obligations may impact our business;

• If we experience greater credit losses than anticipated, earnings may be adversely impacted;

• We are subject to risks and losses resulting from fraudulent activities that could adversely impact our financial performance and results of operations;

• Geographic concentration in our loan portfolio may unfavorably impact our operations;

• Our commercial business and mortgage loans increase our exposure to credit risks;

• Our indirect and consumer lending involves risk elements in addition to normal credit risk;

• Lack of seasoning in portions of our loan portfolio could increase risk of credit defaults in the future;

• We accept deposits that do not have a fixed term, and which may be withdrawn by the customer at any time for any reason;

• We are subject to environmental liability risk associated with our lending activities;

• We operate in a highly competitive industry and market area;

• Legal and regulatory proceedings and related matters could adversely affect us and the banking industry in general;

• Any future FDIC insurance premium increases may adversely affect our earnings;

• We are highly regulated, and any adverse regulatory action may result in additional costs, loss of business opportunities, and reputational damage;

• We are subject to the CRA and fair lending laws, and failure to comply with these laws could lead to material penalties;

• The policies of the Federal Reserve have a significant impact on our earnings;

• Our investment advisory and wealth management operations are subject to risk related to the regulation of the financial services industry and market volatility;

• We make certain assumptions and estimates in preparing our financial statements that may prove to be incorrect, which could significantly impact the results of our operations, cash flows and financial

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condition, and we are subject to new or changing accounting rules and interpretations, and the failure by us to correctly interpret or apply these evolving rules and interpretations could have a material adverse effect;

• The value of our goodwill and other intangible assets may decline in the future;

• We may be unable to successfully implement our growth strategies, including the integration and successful management of newly acquired businesses;

• The introduction of new products and services may subject us to increased regulation and regulatory scrutiny and may affect our reputation;

• Acquisitions may disrupt our business and dilute shareholder value;

• Our tax strategies and the value of our deferred tax assets and liabilities could adversely affect our operating results and regulatory capital ratios;

• Liquidity is essential to our businesses;

• We rely on dividends from our subsidiaries for most of our revenue;

• If our risk management framework does not effectively identify or mitigate our risks, we could suffer losses;

• We face competition in staying current with technological changes and banking alternatives to compete and meet customer demands;

• We rely on other companies to provide key components of our business infrastructure;

• A breach in security of our or third-party information systems, including the occurrence of a cyber incident or a deficiency in cybersecurity, or a failure by us to comply with New York State cybersecurity regulations, may subject us to liability, result in a loss of customer business or damage our brand image;

• The soundness of other financial institutions could adversely affect us;

• We may need to raise additional capital in the future and such capital may not be available on acceptable terms or at all;

• We may not pay or may reduce the dividends on our common stock;

• We may issue debt and equity securities or securities convertible into equity securities, any of which may be senior to our common stock as to distributions and in liquidation, which could dilute our current shareholders or negatively affect the value of our common stock;

• Our certificate of incorporation, our bylaws, and certain banking laws may have an anti-takeover effect;

• The market price of our common stock may fluctuate significantly in response to a number of factors;

• We may not be able to attract and retain skilled people;

• We use financial models for business planning purposes that may not adequately predict future results;

• We depend on the accuracy and completeness of information about or from customers and counterparties;

• Our business may be adversely affected by conditions in the financial markets and economic conditions generally, including macroeconomic pressures such as inflation, supply chain issues, and geopolitical risks associated with international conflict;

• Severe weather, natural disasters, public health emergencies and pandemics, acts of war or terrorism, and other external events could significantly impact our business;

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• our use of the net proceeds from this offering, which may include a balance sheet restructuring through the repositioning of a substantial portion of our available-for-sale debt securities portfolio to support our continued growth as well as the repayment of indebtedness; and

• other factors that are discussed in the section entitled “Risk Factors.”

The foregoing factors could cause results or performance to materially differ from those expressed in our forward-looking statements, should not be considered exhaustive, and should be read together with other cautionary statements that are included in this prospectus supplement, in the accompanying prospectus and in the documents incorporated herein by reference, including in our Annual Report on Form 10-K for the year ended December 31, 2023, in our Quarterly Reports on Form 10-Q for the periods ended March 31, 2024, June 30, 2024 and September 30, 2024, and other filings we may make with the SEC, copies of which are available from us at no charge. New risks and uncertainties may emerge from time to time, and it is not possible for us to predict their occurrence or how they will affect us. If one or more of the factors affecting our forward-looking information and statements proves incorrect, then our actual results, performance, or achievements could differ materially from those expressed in, or implied by, forward-looking information and statements contained in this prospectus supplement. Therefore, we caution you not to place undue reliance on our forward-looking information and statements. We disclaim any duty to revise or update the forward-looking statements, whether written or oral, to reflect actual results or changes in the factors affecting the forward-looking statements, except as specifically required by law.

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PROSPECTUS SUPPLEMENT SUMMARY

The following summary provides a brief overview of certain information appearing elsewhere in this prospectus supplement and the documents incorporated by reference herein, which are described under “Incorporation of Certain Documents by Reference.” Because it is a summary, it does not contain all the information that may be important to you. Before making an investment decision, you should read this entire prospectus supplement and the accompanying prospectus carefully, including the section entitled “Risk Factors” in this prospectus supplement, and the documents incorporated by reference herein and in the accompanying prospectus, including the financial statements and the accompanying notes contained in such documents.

Financial Institutions, Inc.

Financial Institutions, Inc. is a financial holding company organized in 1931 under the laws of New York State. The Company was incorporated on September 15, 1931, but the continuity of the Company’s banking business is traced to the organization of the National Bank of Geneva on March 28, 1817.

The Company’s primary business is the operation of its subsidiaries. It does not engage in any other substantial business activities. The Company’s two direct wholly-owned subsidiaries are: (1) the Bank, which provides a full range of banking services to consumer, commercial and municipal customers in Western and Central New York and commercial loans in the Mid-Atlantic and Central New York regions; and (2) Courier Capital, LLC , which provides customized investment management, investment consulting and retirement plan services to individuals, businesses, institutions, foundations and retirement plans.

Our business strategy has been to maintain a community bank philosophy, which consists of focusing on and understanding the individualized banking and other financial services needs of individuals, municipalities and businesses of the communities surrounding our primary service area. We believe this focus allows us to be more responsive to our customers’ needs and provide a high level of personal service that differentiates us from larger competitors, resulting in long-standing and broad-based banking relationships. Our core customers are primarily small- to medium-sized businesses, individuals and community organizations who prefer to build banking and wealth management relationships with a community bank that offers high-quality, competitively-priced products and services with personalized service. Because of our identity and origin as a locally operated bank, we believe that our level of personal service provides a competitive advantage over larger banks, which tend to consolidate decision-making authority outside local communities.

A key aspect of our current business strategy is to foster a community–oriented culture where our customers and employees establish long-standing and mutually beneficial relationships. We believe that we are well-positioned to be a strong competitor within our market area because of our focus on community banking needs and customer service, our comprehensive suite of deposit, loan, insurance and wealth management products and services typically found at larger banks, our highly experienced management team and our strategically located banking centers.

We prioritize customer acquisition through cost-effective, high-demand digital, virtual and physical channels, while maintaining a community bank distinctiveness relative to larger banks and digital-only neobanks. We leverage the retail branch network and customer contact center to build trust and credibility, provide personal financial education and advice, offer convenience, and bridge digital and physical channels. Our enhanced digital capabilities complement a continued focus on a consistent customer experience and engagement across physical and virtual channels, including using branches to create deeper engagement and relationships with customers, balancing customer engagement with efficiency opportunities (e.g., framing outreach to the customer contact center to teach customers how to use digital channels, in addition to addressing the reason for the call), and maintaining and expanding our customer reach digitally, physically or virtually. By employing digital channels

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across our current products and services, we deepen existing relationships and enter new geographies or market segments that would otherwise be prohibitively expensive targets using traditional approaches. Deepening our existing digital capabilities allows us to capitalize on a shift in customer preferences away from physical branches.

We have evolved to meet changing customer needs by offering complementary physical, digital and virtual channels. We focus on technology to provide solutions that fit our customers’ preferences for transacting business with us. Branches are staffed by certified personal bankers who are trained to meet a broad array of customer needs. Our digital banking capabilities, interactive teller machine functionality and Customer Contact Center provide additional self-serve and phone options through which customer needs are met effectively.

We will continue to explore market expansion opportunities that complement current market areas as opportunities arise. Our primary focus will be on increasing the Bank’s market share within existing markets, while taking advantage of potential growth opportunities within our noninterest income lines of business by acquiring businesses that can be incorporated into existing operations. We believe our capital position remains strong enough to support an active merger and acquisition strategy and the expansion of our core financial service businesses. Consequently, we continue to explore acquisition opportunities in these activities. When evaluating acquisition opportunities, we will balance the potential for earnings accretion with maintaining adequate capital levels, which could result in our common stock being the predominant form of consideration and/or the need for us to raise capital.

Conversations with potential strategic partners occur on a regular basis. The evaluation of any potential opportunity will favor a transaction that complements our core competencies and strategic intent, with a lesser emphasis being placed on geographic location or size. Additionally, we remain committed to maintaining a diversified revenue stream. Our senior management team has experience in acquisitions and post-acquisition integration of operations and is prepared to act promptly should a potential opportunity arise but will remain disciplined with its approach. We believe this experience positions us to successfully acquire and integrate additional financial services and banking businesses.

We are regulated by the NYSDFS, the Federal Reserve, and other regulatory agencies. See the section entitled “Supervision and Regulation” in our Annual Report on Form 10-K for the year ended December 31, 2023 for more information.

At September 30, 2024, the Company had consolidated total assets of $6.16 billion, deposits of $5.31 billion and shareholders’ equity of $500.3 million.

Our principal executive office is located at 220 Liberty Street, Warsaw, New York 14569 and our telephone number is (585) 786-1100. Our common stock is traded on Nasdaq under the ticker symbol “FISI.” Additional information about us and our subsidiaries may be found in the documents incorporated by reference into this prospectus supplement. See “Where You Can Find More Information.”

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THE OFFERING

Issuer Financial Institutions, Inc.

Shares of common stock offered by us 4,000,000 shares (or 4,600,000 shares if the underwriters exercise in full their option to purchase additional shares).

Shares of common stock to be outstanding after the offering 19,474,314 shares (or 20,074,314 shares if the underwriters exercise in full their option to purchase additional shares).

Public offering price per share $25.00.

Use of proceeds We intend to use the net proceeds from this offering for general corporate purposes which may include a balance sheet restructuring through the repositioning of a substantial portion of our available-for-sale debt securities portfolio to support our continued growth as well as the repayment of indebtedness. See “Use of Proceeds.”

Indication of Interest Certain of our directors, executive officers and their related persons or entities have indicated an interest in purchasing a portion of the shares of common stock in this offering at a price equal to the public offering price. Because this indication of interest is not a binding agreement or commitment to purchase, these persons could determine to purchase more, less or no shares in this offering or the underwriters could determine to sell more, less or no shares to these persons.

Risk factors You should carefully read and consider the information set forth under the heading “Risk Factors” beginning on page S-9 in this prospectus supplement and the accompanying prospectus, and under the heading “Part I. Item IA. Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2023, which is incorporated by reference into this prospectus supplement, along with all other information included in and incorporated by reference into this prospectus supplement and the accompanying prospectus before deciding to invest in our common stock.

Lock-up agreements We, each of our executive officers, and directors have entered into lock-up agreements, which restrict such persons from engaging in certain transactions in our securities during the Lock-Up Period (as defined below) without the consent of the underwriters. See “Underwriting—Lock-Up Agreements” in this prospectus supplement.

Listing and trading symbol Our common stock is traded on Nasdaq under the symbol “FISI”.

Unless otherwise noted, references in this prospectus supplement to the number of shares of our common stock outstanding after this offering are based on 15,474,314 shares of our common stock issued and outstanding as of September 30, 2024. Except as otherwise indicated, the information in this prospectus supplement:

• includes 10,000 unvested restricted stock shares outstanding as of September 30, 2024;

• excludes 107,346 unvested restricted stock or performance stock units outstanding as of September 30, 2024;

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• excludes 281,898 shares of our common stock reserved for issuance under the Financial Institutions, Inc. Amended and Restated 2015 Long-Term Incentive Plan, as of September 30, 2024;

• assumes no exercise of the underwriters’ option to purchase additional shares of our common stock; and

• does not attribute to any director or executive officer any purchases of shares of our common stock in this offering.

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RISK FACTORS

An investment in our common stock involves a significant degree of risk. The material risks and uncertainties that management believes affect us are described below. Before you decide to invest in our common stock, you should carefully read and consider all of the information contained in this prospectus supplement, including the risks and uncertainties described below and under “Special Cautionary Note Regarding Forward-Looking Statements” in this prospectus supplement, and the other documents incorporated by reference into this prospectus supplement, including the risks and uncertainties described under “Risk Factors” in the accompanying prospectus, our Annual Report on Form 10-K for the year ended December 31, 2023, and our Quarterly Reports on Form 10-Q for the quarters ended March 31, 2024, June 30, 2024 and September 30, 2024. Any of these risks, if they are realized, could have an adverse effect on our business, financial condition and results of operations, and consequently, the value of our common stock. In any such case, you could lose all or a portion of your original investment. Further, additional risks and uncertainties not currently known to us or that we currently believe to be immaterial may also adversely affect us. This prospectus supplement also contains forward-looking statements that involve risks and uncertainties. See the section entitled “Special Cautionary Note Regarding Forward-Looking Statements.”

Risks Related to an Investment in Our Common Stock and the Offering

Future sales or the possibility of future sales of our common stock could depress the market price of our common stock.

Our amended and restated certificate of incorporation authorizes us to issue up to 50,000,000 shares of common stock. Following the completion of this offering, we will have issued and outstanding 19,474,314 shares of our common stock (or 20,074,314 shares if the underwriters exercise their option to purchase additional shares in full), a significant portion of which will be freely transferable without restriction or further registration under the Securities Act. We and our executive officers and our directors holding, in the aggregate, 443,194 shares of our common stock as of September 30, 2024 (representing approximately 2.86% of our outstanding shares of common stock as of such date), have agreed not to sell any shares of our common stock for a period of 90 days from the date of this prospectus supplement, or the Lock-Up Period, subject to certain exceptions. See the section entitled “Underwriting.” Following the expiration of the Lock-Up Period, all of these shares will be eligible for resale under Rule 144 of the Securities Act, subject to any applicable holding period requirements and volume limitations. In addition, the underwriters, at any time and without notice, may release all or any portion of the common stock subject to such lock-up restrictions. The remaining shares of our common stock outstanding prior to this offering are not subject to lock-up agreements and they may be freely resold at any time, subject to the restrictions described in this prospectus supplement, if applicable, for affiliate holders. The shares of our common stock being offered and sold in this offering will also generally be available for resale into the public markets. See the section entitled “Underwriting.”

We may also issue additional shares in the future for various purposes without any action or approval by our shareholders, and these shares, once issued, will be available for sale into the public market, subject to the restrictions described in this prospectus supplement, if applicable, for affiliate holders. The market price for our common stock may decline significantly when the restrictions on resale by our existing shareholders lapse. Actual or anticipated issuances or sales of substantial additional amounts of our common stock following this offering could cause the market price of our common stock to decline significantly and make it more difficult for us to sell equity or equity-related securities in the future on favorable terms, or at all. We cannot predict the size of future issuances of our common stock or the effect, if any, that future issuances and sales of our common stock will have on the market price of our common stock. Sales of substantial amounts of our common stock, or the perception that such sales could occur, may adversely affect prevailing market prices for our common stock. Further, a decline in the market price of our common stock might impede our ability to raise capital through the issuance of additional common stock or other equity securities.

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The market price of our common stock may be volatile, and we may not be able to meet investor or analyst expectations. You may not be able to resell your shares at or above the price you paid and may lose part or all of your investment as a result.

Our stock price has fluctuated from a low of $13.84 to a high of $34.43 between January 1, 2022 and December 6, 2024. Volatility in the market price of our common stock may negatively impact the price at which our common stock may be sold and may also negatively impact the timing of any sale. The market price of our common stock may continue to fluctuate widely in response to a variety of factors including the risk factors described herein and, among other things:

• actual or anticipated variations in quarterly or annual operating results, financial conditions, or credit quality;

• changes in business or economic conditions;

• changes in accounting standards, policies, guidance, interpretations, or principles;

• changes in recommendations or research reports about us or the financial services industry in general published by securities analysts;

• the failure of securities analysts to cover, or to continue to cover, us;

• changes in financial estimates or publication of research reports and recommendations by financial analysts or actions taken by rating agencies with respect to us or other financial institutions;

• news reports relating to trends, concerns, and other issues in the financial services industry;

• reports related to the impact of natural or man-made disasters in our market;

• perceptions in the marketplace regarding us and or our competitors;

• sudden increases in the demand for our common stock, including as a result of any “short squeezes”;

• significant acquisitions or business combinations, strategic partnerships, joint ventures, or capital commitments by or involving us or our competitors;

• additional investments from third parties;

• additions or departures of key personnel;

• future sales or issuance of additional shares of our common stock;

• fluctuations in the market price of our competitors’ common stock and the operating results of our competitors;

• changes or proposed changes in laws or regulations, or differing interpretations thereof, affecting our business, or enforcement of these laws or regulations;

• new technology used, or services offered, by competitors;

• additional investments from third parties; or

• geopolitical conditions such as acts or threats of terrorism, pandemics, or military conflicts.

In particular, the realization of any of the risks described in this section or under the heading “Part I. Item IA. Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2023 which is incorporated herein by reference, could have an adverse effect on the market price of our common stock and cause the value of your investment to decline. In addition, the stock market in general has experienced extreme volatility that has often been unrelated to the operating performance of particular companies. These broad market fluctuations may adversely affect the market price of our common stock over the short, medium or long term, regardless of our actual performance. If the market price of our common stock reaches an elevated level

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following this offering, it may materially and rapidly decline. In the past, following periods of volatility in the market price of a company’s securities, shareholders have often instituted securities class action litigation. If we were to be involved in a class action lawsuit, it could divert the attention of our senior management and could adversely affect our business, financial condition and results of operations.

If securities or industry analysts do not publish research or publish inaccurate or unfavorable research about our business, or change their recommendations regarding our common stock or if our operating results do not meet their expectations, the market price of our common stock and trading volume could decline.

The trading market for our common stock depends in part on the research and reports that securities or industry analysts publish about us or our business. If one or more of the analysts who covers us downgrades our common stock or publishes inaccurate or unfavorable research about our business, or our operating results do not meet their expectations, either absolutely or relative to our competitors, the market price of our common stock would likely decline. If one or more of these analysts cease coverage of us or fail to publish reports about us regularly, (i) we could lose visibility in the financial markets, which in turn could cause the market price of our common stock or trading volume to decline and (ii) demand for our common stock could decrease, which could cause the market price of our common stock and trading volume to decline. If we fail to meet the expectations of analysts for our operating results, the market price of our common stock would likely decline.

We depend on the Bank for cash flow, and the Bank’s ability to make cash distributions is restricted, which could impact our ability to satisfy our obligations.

We are a bank holding company with no material activities other than activities incidental to holding the common stock of the Bank. Our principal source of funds to pay distributions on our common stock and service any of our obligations, other than further issuances of securities, is dividends received from the Bank. The holding company, Financial Institutions, Inc., is a legal entity separate and distinct from the Bank. Furthermore, the Bank is not obligated to pay dividends to us, and any dividends paid to us would depend on the earnings or financial condition of the Bank, various business considerations, and applicable law and regulation. As is generally the case for banking institutions, the profitability of the Bank is subject to the fluctuating cost and availability of money, changes in interest rates, and economic conditions in general. In addition, various federal statutes and regulations limit the amount of dividends that the Bank may pay to us without regulatory approval.

Our dividend policy may change without notice, we may not pay dividends on our common stock in the future, and our ability to pay dividends is subject to certain restrictions.

We have paid cash dividends on our common stock in every quarter since September 2013. On August 19, 2024, we announced the declaration of a quarterly cash dividend of $0.30 per share on our outstanding common stock. The dividend was paid on October 2, 2024 to shareholders of record as of the close of business on September 13, 2024.

Holders of our common stock are entitled to receive only such dividends as our board of directors may declare out of funds legally available for such payments. Our board of directors may, in its sole discretion, change the amount or frequency of dividends or discontinue the payment of dividends entirely. The timing, declaration, amount and payment of future cash dividends, if any, will be within the discretion of our board of directors and will depend upon then-existing conditions, including our results of operations, financial condition, capital requirements, investment opportunities, growth opportunities, any legal, regulatory, contractual or other limitations on our ability to pay dividends and other factors our board of directors may deem relevant. In addition, we are a bank holding company, and our ability to declare and pay dividends is dependent on federal regulatory considerations, including the guidelines of the Federal Reserve regarding capital adequacy and dividends. It is the policy of the Federal Reserve that bank holding companies should generally pay dividends on common stock only out of earnings, and only if prospective earnings retention is consistent with the organization’s expected future needs, asset quality and financial condition, and that bank holding companies

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should inform and consult with the Federal Reserve in advance of declaring and paying a dividend that exceeds earnings for the period for which the dividend is being paid. If the Company effectuates a balance sheet restructuring transaction, it is possible that the Company would be required to seek the Federal Reserve’s non-objection prior to declaring dividends. Any future payment of dividends will also depend on the Bank’s ability to make distributions and payments to the Company, as these distributions and payments are our principal source of funds to pay dividends. The Bank is also subject to various legal, regulatory and other restrictions on its ability to make distributions and payments to the Company. In addition, in the future, we may enter into borrowing or other contractual arrangements that restrict our ability to pay dividends. As a consequence of these various limitations and restrictions, we may not be able to make, or may have to reduce or eliminate, the payment of dividends on our common stock. Any change in the level of our dividends or the suspension of the payment thereof could have an adverse effect on the market price of our common stock. See also “Item 1. Business—Supervision and Regulation—Dividends” in our Annual Report on Form 10-K for the year ended December 31, 2023, which is incorporated by reference into this prospectus supplement.

The holders of our debt obligations and preferred stock will have priority over our common stock with respect to payment in the event of liquidation, dissolution or winding up and with respect to the payment of interest and dividends.

In any liquidation, dissolution or winding up of the Company, our common stock would rank below all claims of debt holders against us as well as any preferred stock that has been issued. As of September 30, 2024, we had outstanding an aggregate of $75 million of subordinated notes, 1,435 shares of outstanding Class A Preferred Stock, the Series A 3% Preferred Stock, and 171,486 shares of outstanding Class B Preferred Stock, the Series B-1 8.48% Preferred Stock. We could also incur future debt obligations or issue preferred stock in the future to raise additional capital. In such event, holders of our common stock will not be entitled to receive any payment or other distribution of assets upon the liquidation, dissolution or winding up of the Company until after all of our obligations to the debt holders are satisfied and holders of subordinated debt and senior equity securities, including preferred shares, if any, have received any payment or distribution due to them. In addition, we may be required to pay interest on future debt obligations and preferred stock before we would be able to pay any dividends on our common stock. Since any decision to issue debt securities or incur other borrowings in the future will depend on market conditions and other factors beyond our control, the amount, timing, nature or success of our future capital raising efforts is uncertain. Thus, holders of our common stock bear the risk that our future issuances of debt securities or our incurrence of other borrowings will negatively affect the market price of our common stock.

Our management will have broad discretion in allocating the net proceeds of the offering. Our failure to effectively utilize such net proceeds may have an adverse effect on our financial performance and the value of our common stock.

We intend to use the net proceeds from this offering for general corporate purposes which may include a balance sheet restructuring through the repositioning of a substantial portion of our available-for-sale debt securities portfolio to support our continued growth as well as the repayment of indebtedness. The timing and execution of the restructuring will be dependent on market conditions. However, we are not required to apply any portion of the net proceeds of this offering for any particular purpose and our management could use the proceeds for purposes other than those contemplated at the time of this offering. Accordingly, our management will have broad discretion in the application of the net proceeds from this offering, and you will be relying on the judgment of our management regarding the application of these proceeds. You will not have the opportunity, as part of your investment decision, to assess whether we are using the proceeds appropriately. Our management might not apply our net proceeds in ways that ultimately increase the value of your investment. If we do not invest or apply the net proceeds from this offering in ways that enhance shareholder value, we may fail to achieve expected financial results, which could cause the market price of our stock to decline.

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New York law and the provisions of our amended and restated certificate of incorporation and amended and restated bylaws may have an anti-takeover effect, and there are substantial regulatory limitations on changes of control of bank holding companies.

Certain provisions of our amended and restated certificate of incorporation and amended and restated bylaws, as well as New York law, and the Bank Holding Company Act, and Change in Bank Control Act, could delay or prevent a change of control that you may favor. Our amended and restated certificate of incorporation and amended and restated bylaws include certain provisions that could delay a takeover or change in control of us, including: a staggered board of directors, the exclusive right of our board to fill any director vacancy; and advance notice requirements for shareholder proposals and director nominations.

The business combination provisions of the New York Business Corporation Law could prohibit or delay mergers or other takeovers or change in control attempts with respect to the Company and, accordingly, may discourage attempts to acquire the Company. In general such provisions prohibit an “interested shareholder” (i.e., a person who owns 20% or more of our outstanding voting stock) from engaging in various business combination transactions with our company, unless (a) the business combination transaction, or the transaction in which the interested shareholder became an interested shareholder, was approved by the Board of Directors prior to the interested shareholder’s stock acquisition date, (b) the business combination transaction was approved by the disinterested shareholders at a meeting called no earlier than five years after the interested shareholder’s stock acquisition date, or (c) if the business combination transaction takes place no earlier than five years after the interested stockholder’s stock acquisition date, the price paid to all the stockholders under such transaction meets statutory criteria.

If you purchase our common stock in this offering, you may incur immediate and substantial dilution in the book value of your shares.

If you purchase shares in this offering, the value of your shares based on our actual book value will immediately be less than the offering price you paid. As a result, investors purchasing stock in this offering may receive significantly less than the purchase price paid in this offering in the event of liquidation.

An investment in our common stock is not an insured deposit and is not guaranteed by the FDIC, so you could lose some or all of your investment.

An investment in our common stock is not a bank deposit and, therefore, is not insured against loss or guaranteed by the FDIC, any other deposit insurance fund or by any other public or private entity. Investment in our common stock is inherently risky for the reasons described herein, and is subject to the same market forces that affect the price of common stock in any company. As a result, if you acquire our common stock, you could lose some or all of your investment.

Risks Relating to Our Business and our Industry and Regulation

For risks associated with our business and industry, see the section entitled “Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2023 which is incorporated into this prospectus supplement by reference, as the same may be updated from time to time prior to the completion of this offering by our future filings under the Exchange Act.

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USE OF PROCEEDS

We estimate that the net proceeds to us from this offering will be approximately $94.3 million, or approximately $108.6 million if the underwriters exercise in full their option to purchase additional shares, in each case, after deducting underwriting discounts and commissions and the estimated offering expenses payable by us.

We intend to use the net proceeds from this offering for general corporate purposes which may include a balance sheet restructuring through the repositioning of a substantial portion of our available-for-sale debt securities portfolio to support our continued growth as well as the repayment of indebtedness. The timing and execution of the restructuring will be dependent on market conditions, although we may complete the restructuring before the end of the fourth quarter of 2024. We expect that this restructuring will involve the following:

• The sale of debt securities with a fair value between approximately $550 to $650 million, which carry an average yield of approximately 1.75% and a weighted average life of approximately 5.5 years as of September 30, 2024;

• Based upon our current estimates of market pricing, we expect to recognize a pre-tax loss related to the restructuring of approximately $100 million which may occur in the fourth quarter of 2024; and

• The subsequent purchase of debt securities with current-market yields. We expect to purchase approximately $550 to $650 million of new debt securities to be classified as available-for-sale securities. Based on current market rates, we estimate the new securities would have an estimated aggregate yield of approximately 5.0%, but the actual yield will depend on market conditions at the time of purchase.

Our board of directors and management will have broad discretion in the application of the net proceeds from this offering, and investors will be relying on the judgment of our board of directors and management with regard to the use of these net proceeds. Pending the use of the net proceeds from this offering as described above, we may invest the net proceeds in short-term liquid instruments.

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CAPITALIZATION

The following table sets forth our capitalization as of September 30, 2024:

• on an actual basis; and

• on an as adjusted basis to reflect the sale of 4,000,000 shares of our common stock offered by us at the public offering price of $25.00 per share in this offering, after deducting the underwriting discount and our estimated offering expenses (assuming the underwriters’ option to purchase additional shares from us is not exercised).

This table should be read in conjunction with, and is qualified in its entirety by reference to, the information appearing under “Use of Proceeds” included elsewhere in this prospectus supplement, our historical financial statements and related notes incorporated by reference into this prospectus supplement.

	As of September 30, 2024
(Dollars in thousands)	Actual			As Adjusted (1)
Assets:
Cash and due from banks		249,569			343,869
Long-term debt:
Federal Home Loan Bank advances		50,000			50,000
Subordinated debentures due 2030, net		74,765			74,765
Total long-term debt		124,765			124,765
Shareholders’ equity:
Series A 3% preferred stock, $100 par value; 1,533 shares authorized; 1,435 shares issued		143			143
Series B-1 8.48% preferred stock, $100 par value; 200,000 shares authorized; 171,486 shares issued		17,149			17,149
Common stock, $0.01 par value; 50,000,000 shares authorized; 15,474,314 shares issued and outstanding - actual and 19,474,314 shares issued and outstanding - as adjusted		161			201
Additional paid-in capital		125,112			219,372
Retained earnings		477,861			477,861
Accumulated other comprehensive loss		(102,029	)		(102,029	)
Treasury stock at cost		(18,055	)		(18,055	)
Total shareholders’ equity	$	500,342		$	594,642
Total capitalization	$	625,107		$	719,407
Capital ratios: (2)
Total capital (to risk-weighted assets)		12.95	%		14.79	%
Tier 1 capital (to risk-weighted assets)		10.62	%		12.45	%
Common equity Tier 1 capital (to risk-weighted assets)		10.28	%		12.11	%
Tier 1 leverage		8.98	%		10.53	%

(1) Does not give effect to the application of the net proceeds of this offering.

(2) The as adjusted calculations for the risk-based capital ratios for the Company assume that the net proceeds from the sale of the common stock are invested in assets that carry a 0% risk weighting.

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MATERIAL U.S. FEDERAL INCOME TAX CONSIDERATIONS TO NON-U.S. HOLDERS

The following discussion is a summary of the material U.S. federal income tax consequences of the purchase, ownership and disposition of our common stock by Non-U.S. Holders (as defined below) that acquire our common stock in this offering and hold it as a capital asset (generally, property held for investment). This discussion does not purport to be a complete analysis of all potential tax effects. The effects of other U.S. federal tax laws, such as estate and gift tax laws, and any applicable state, local or non-U.S. tax laws are not discussed. This discussion is based on the United States Internal Revenue Code of 1986, as amended, or the Code, Treasury Regulations promulgated thereunder, judicial decisions, and published rulings and administrative pronouncements of the Internal Revenue Service, or the IRS, in each case in effect as of the date of this prospectus supplement. These authorities may change or be subject to differing interpretations at any time. Any such change or differing interpretation may be applied retroactively in a manner that could adversely affect a Non-U.S. Holder of our common stock. We have not sought and will not seek any rulings from the IRS regarding the statements made and the conclusions reached in the discussion below. There can be no assurance the IRS or a court will agree with our position discussed below regarding the U.S. federal income tax consequences of the purchase, ownership and disposition of our common stock. We cannot assure you that a change in law will not significantly alter the tax considerations described in this discussion.

This discussion does not address all U.S. federal income tax consequences relevant to a Non-U.S. Holder’s particular circumstances, including the alternative minimum tax and the impact of the Medicare contribution tax on net investment income. In addition, it does not address consequences relevant to Non-U.S. Holders subject to special rules, including, without limitation:

• U.S. expatriates and former citizens or long-term residents of the United States;

• persons who have elected to mark securities to market or who hold our common stock as part of a hedge, straddle or other risk reduction strategy or as part of a conversion transaction or other integrated investment;

• banks, insurance companies, and other financial institutions;

• brokers, dealers or traders in securities;

• corporations that accumulate earnings to avoid U.S. federal income tax;

• tax-exempt organizations, pension plans, tax-qualified retirement plans, or governmental organizations; and

• persons deemed to sell our common stock under the constructive sale provisions of the Code.

If an entity treated as a partnership for U.S. federal income tax purposes holds our common stock, the tax treatment of a partner in the partnership will depend on the status of the partner and the activities of the partnership. Accordingly, partnerships holding our common stock and the partners in such partnerships should consult their tax advisors regarding the U.S. federal income tax consequences to them.

THIS DISCUSSION IS FOR INFORMATION PURPOSES ONLY AND IS NOT TAX ADVICE. INVESTORS SHOULD CONSULT THEIR TAX ADVISORS WITH RESPECT TO THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO THEIR PARTICULAR SITUATIONS AS WELL AS ANY TAX CONSEQUENCES OF THE PURCHASE, OWNERSHIP AND DISPOSITION OF OUR COMMON STOCK ARISING UNDER THE U.S. FEDERAL ESTATE OR GIFT TAX LAWS OR UNDER THE LAWS OF ANY STATE, LOCAL OR NON-U.S. TAXING JURISDICTION OR UNDER ANY APPLICABLE INCOME TAX TREATY.

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Definition of a Non-U.S. Holder

For purposes of this discussion, a “Non-U.S. Holder” is any beneficial owner of our common stock that is for U.S. federal income tax purposes:

• a non-resident alien;

• a foreign corporation (or any other entity treated as a corporation for U.S. federal income tax purposes);

• an estate, the income of which is not subject to U.S. federal income taxation regardless of its source; or

• a trust that does not have in effect a valid election under the U.S. Treasury Regulations, to be treated as a United States person and either (i) no court within the United States is able to exercise primary supervision over the trust’s administration or (ii) no “United States person” (within the meaning of Section 7701(a)(30) of the Code) has the authority to control all substantial decisions of that trust.

Distributions on Common Stock

Distributions of cash or property on our common stock will generally constitute dividends for U.S. federal income tax purposes to the extent of our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Amounts not treated as dividends for U.S. federal income tax purposes will first constitute a tax-free return of capital to the extent of a Non-U.S. Holder’s adjusted tax basis in its common stock and thereafter capital gain, which is subject to the tax treatment described below in the section entitled “—Sale or Other Taxable Disposition”.

Subject to the discussion below in the sections entitled “—Information Reporting and Backup Withholding” and “—FATCA Withholding” and the discussion below on effectively connected income, dividends paid to a Non-U.S. Holder will generally be subject to U.S. federal withholding tax at a rate of 30% of the gross amount of the dividends or such lower rate specified by an applicable income tax treaty. To receive a reduced withholding rate, a Non-U.S. Holder must furnish a valid IRS Form W-8BEN or W-8BEN-E (or other applicable documentation) certifying qualification for the lower treaty rate. A Non-U.S. Holder that holds our common stock through a financial institution or other agent will be required to provide appropriate documentation to the financial institution or other agent, which then will be required to provide certification to us or our paying agent either directly or through other intermediaries. A Non-U.S. Holder that does not timely furnish the required documentation, but that qualifies for a reduced income tax treaty rate, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS.

If dividends paid to a Non-U.S. Holder are effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the United States (and, as provided by an applicable income tax treaty, attributable to a permanent establishment or fixed base maintained by the Non-U.S. Holder in the United States), the Non-U.S. Holder will be exempt from the U.S. federal withholding tax described above. To claim the exemption, the Non-U.S. Holder must furnish to the applicable withholding agent a valid IRS Form W-8ECI, certifying that the dividends are effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the United States.

Any such effectively connected dividends will be subject to U.S. federal income tax on a net income basis at the regular rates that also apply to U.S. persons. A Non-U.S. Holder that is a corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on such effectively connected dividends, as adjusted for certain items. Non-U.S. Holders should consult their tax advisors regarding any applicable tax treaties that may provide for different rules.

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Sale or Other Taxable Disposition

Subject to the discussion below under “–Information Reporting and Backup Withholding” and “–FATCA Withholding,” a Non-U.S. Holder generally will not be subject to U.S. federal income tax or withholding tax on any gain realized upon the sale or other taxable disposition of our common stock unless:

• the gain is effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the United States (and, as provided by an applicable income tax treaty, attributable to a permanent establishment or fixed base maintained by the Non-U.S. Holder in the United States);

• the Non-U.S. Holder is a nonresident alien individual present in the United States for 183 days or more during the taxable year of the disposition and certain other requirements are met; or

• we are or have been a U.S. real property holding corporation, or USRPHC, for U.S. federal income tax purposes at any time within the shorter of the five-year period preceding such disposition or such Non-U.S. Holder’s holding period for our common stock, or the relevant period, and the Non-U.S. Holder (i) disposes of our common stock during a calendar year when our common stock is no longer regularly traded on an established securities market or (ii) owned (directly, indirectly and constructively) more than 5% of our common stock at any time during the relevant period.

Gain described in the first bullet point above generally will be subject to U.S. federal income tax on a net income basis at the regular rates in the same manner as if such Non-U.S. Holder were a resident of the United States. A Non-U.S. Holder that is a corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on such effectively connected gain, as adjusted for certain items.

Gain described in the second bullet point above will be subject to U.S. federal income tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty), which may be offset by U.S. source capital losses of the Non-U.S. Holder for the year, provided the Non-U.S. Holder has timely filed U.S. federal income tax returns with respect to such losses.

With respect to the third bullet point above, we believe we currently are not, and do not anticipate becoming, a USRPHC. Because the determination of whether we are a USRPHC depends, however, on the fair market value of our United States real property interests as defined in the Code relative to the fair market value of our non-U.S. real property interests and our other business assets, there can be no assurance we currently are not a USRPHC or will not become one in the future. Gain from a disposition of our common stock described in the third bullet point above will be subject to tax generally as if the gain were effectively connected with the conduct of a trade or business in the United States, except that the “branch profits tax” will not apply.

Information Reporting and Backup Withholding

Payments of dividends on our common stock and the payment of the proceeds from the sale of our common stock effected at a U.S. office of a broker generally will not be subject to backup withholding and the payment of proceeds from the sale of our common stock effected at a U.S. office of a broker will generally not be subject to information reporting, provided the applicable withholding agent does not have actual knowledge or reason to know the Non-U.S. Holder is a United States person and the Non-U.S. Holder either certifies its non-U.S. status, such as by furnishing a valid IRS Form W-8BEN or W-8BEN-E or other documentation upon which it may rely to treat the payments as made to a non-U.S. person in accordance with Treasury Regulations or otherwise establishes an exemption.

However, we are required to file information returns with the IRS in connection with any distribution on our common stock paid to the Non-U.S. Holder, regardless of whether any tax was actually withheld. Copies of information returns that are filed with the IRS may also be made available under the provisions of an applicable treaty or agreement to the tax authorities of the country in which the Non-U.S. Holder resides or is established.

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Payment of the proceeds from the sale of our common stock effected at a foreign office of a broker generally will not be subject to information reporting or backup withholding. However, a sale of our common stock by a Non-U.S. Holder that is effected at a foreign office of a broker will be subject to information reporting and backup withholding if (i) the proceeds are transferred to an account maintained by the Non-U.S. Holder in the United States, (ii) the payment of proceeds or the confirmation of the sale is mailed to the Non-U.S. Holder at a U.S. address or (iii) the sale has some other specified connection with the United States as provided in the Treasury Regulations, unless, in each case, the broker does not have actual knowledge or reason to know that the holder is a United States person and the documentation requirements described above are met or the Non-U.S. Holder otherwise establishes an exemption.

In addition, a sale of our common stock will be subject to information reporting if it is effected at a foreign office of a broker that is (i) a United States person, (ii) a “controlled foreign corporation” for U.S. federal income tax purposes, (iii) a foreign person 50% or more of whose gross income is effectively connected with the conduct of a U.S. trade or business for a specified three-year period or (iv) a foreign partnership, if at any time during its tax year (a) one or more of its partners are “U.S. persons,” as defined in the Treasury Regulations, who in the aggregate hold more than 50% of the income or capital interest in the partnership or (b) such foreign partnership is engaged in the conduct of a trade or business in the United States, in each case unless the broker does not have actual knowledge or reason to know that the holder is a United States person and the documentation requirements described above are met or an exemption is otherwise established. Backup withholding will apply if the sale is subject to information reporting and the broker has actual knowledge that the holder is a United States person.

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against a Non-U.S. Holder’s U.S. federal income tax liability, provided the required information is timely furnished to the IRS.

FATCA Withholding

Sections 1471 through 1474 of the Code and the Treasury Regulations issued thereunder (commonly referred to as the Foreign Account Tax Compliance Act, or FATCA) impose a 30% withholding tax on dividends paid on our shares to, and (subject to the proposed Treasury Regulations discussed below) the gross proceeds derived from the sale or other disposition of our shares by, a foreign entity if the foreign entity is:

• a “foreign financial institution” (as defined under FATCA) that does not furnish proper documentation, typically on IRS Form W-8BEN-E, evidencing either (i) an exemption from FATCA withholding or (ii) its compliance (or deemed compliance) with specified due diligence, reporting, withholding and certification obligations under FATCA or (iii) residence in a jurisdiction that has entered into an intergovernmental agreement with the United States relating to FATCA and compliance with the diligence and reporting requirements of the intergovernmental agreement and local implementing rules; or

• a “non-financial foreign entity” (as defined under FATCA) that does not provide sufficient documentation, typically on IRS Form W-8BEN-E, evidencing either (i) an exemption from FATCA or (ii) adequate information regarding substantial United States beneficial owners of such entity (if any).

Withholding agents may, however, rely on proposed U.S. Treasury Regulations that would no longer require FATCA withholding on payments of gross proceeds. A withholding agent such as a broker, and not the Bank, will determine whether or not to implement gross proceeds FATCA withholding.

If a dividend payment is subject to withholding both under FATCA and the withholding tax rules discussed above in the section entitled “—Distributions on Common Stock,” the withholding under FATCA may be credited against, and therefore reduce, such other withholding tax. Non-U.S. Holders should consult their own tax advisors regarding these requirements and whether they may be relevant to their ownership and disposition of the shares.

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Under certain circumstances, a Non-U.S. Holder will be eligible for refunds or credits of withholding taxes imposed under FATCA by filing a United States federal income tax return. Prospective investors should consult their tax advisors regarding the effect of FATCA on their ownership and disposition of our shares.

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UNDERWRITING

We have entered into an underwriting agreement with Keefe, Bruyette & Woods, Inc., as representative of the several underwriters listed below (collectively, the “underwriters”), with respect to the shares of our common stock being offered pursuant to this prospectus supplement. Subject to certain conditions, we will agree to sell, and each underwriter will severally and not jointly agree to purchase, the number of shares of our common stock indicated in the following table:

	Number of Shares
Keefe, Bruyette & Woods, Inc.		3,600,000
Piper Sandler & Co.		400,000
Total		4,000,000

The underwriters are offering the shares of our common stock subject to a number of conditions, including receipt and acceptance of our common stock by the underwriters. The obligations of the underwriters to pay for and accept delivery of the shares offered by this prospectus supplement are subject to these conditions. The underwriting agreement between us and the underwriters provides that if any underwriter defaults, the purchase commitments of the non-defaulting underwriters may be increased or this offering may be terminated.

In connection with this offering, the underwriters or securities dealers may distribute offering documents to investors electronically. See the section entitled “—Electronic Distribution.”

Underwriting Discount

Shares of our common stock sold by the underwriters to the public will be offered at the public offering price set forth on the cover of this prospectus supplement. Any shares of our common stock sold by the underwriters to securities dealers may be sold at a discount of up to $0.75 per share from the public offering price. Any of these securities dealers may resell any shares of our common stock purchased from the underwriters to other brokers or dealers at a discount of up to $0.25 per share from the public offering price. If all of the shares of our common stock are not sold at the public offering price, the representative may change the offering price and the other selling terms. Sales of shares of our common stock made outside of the United States may be made by affiliates of the underwriters. The underwriters reserve the right to reject an order for the purchase of shares, in whole or in part.

The underwriting fee is equal to the public offering price per share of common stock less the amount paid by the underwriters to us per share of common stock. The underwriting fee is $1.25 per share. The following table shows the per share and total underwriting discounts to be paid to the underwriters assuming both no exercise and full exercise of the underwriters’ option to purchase additional shares.

	Full Exercise of option to purchase additional shares		No Exercise of option to purchase additional shares
Per share	$	1.25	$	1.25
Total	$	5,750,000	$	5,000,000

We estimate the expenses of this offering, not including the underwriting discount, to be approximately $550,000, and such expenses are payable by us. We also have agreed to reimburse the underwriters for their expenses incurred in connection with the offering in an amount up to $150,000.

Indication of Interest

Certain of our directors, executive officers and their related persons or entities have indicated an interest in purchasing a portion of the shares of common stock in this offering at a price equal to the public offering price.

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Because this indication of interest is not a binding agreement or commitment to purchase, these persons could determine to purchase more, less or no shares in this offering or the underwriters could determine to sell more, less or no shares to these persons.

Option to Purchase Additional Shares

We have granted the underwriters an option to purchase up to 600,000 additional shares of our common stock, at the public offering price set forth on the cover page of this prospectus supplement, less the underwriting discount. The underwriters may exercise this option, in whole or in part, from time to time for a period of 30 days from the date of this prospectus supplement. If the underwriters exercise this option, each underwriter will be obligated, subject to the conditions in the underwriting agreement, to purchase a number of additional shares of our common stock proportionate to the number of shares reflected next to such underwriter’s name in the table above relative to the total number of shares reflected in such table.

Lock-Up Agreements

We, our executive officers and directors are entering into lock-up agreements with the underwriters. Under these agreements, we and each of these persons may not, without the prior written approval of the representative and subject to certain exceptions:

• offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, or otherwise directly or indirectly dispose of or transfer any shares of our common stock or any securities convertible into or exchangeable or exercisable for our common stock, whether now owned or hereafter acquired or with respect to which such person has or hereafter acquires the power of disposition, or exercise any right with respect to the registration thereof, or file with the Securities Exchange Commission any registration statement under the Securities Act, relating to any, shares of common stock or any such other securities;

• enter into any swap, hedge, or other agreement that transfers, in whole or in part, the economic consequence of ownership of the shares of our common stock or such other securities; or

• publicly disclose the intention to make any such offer, pledge, sale or disposition, or to enter into any such swap, hedge, transaction or arrangement.

These restrictions are subject to customary exceptions and will be in effect for a period of 90 days after the date of this prospectus supplement. At any time and without public notice, the representative may, in its sole discretion, waive or release all or some of the securities from these lock-up agreements.

These restrictions also apply to securities convertible into or exchangeable or exercisable for or repayable with our common stock to the same extent as they apply to our common stock. They also apply to common stock owned now or later acquired by the person executing the agreement or for which the person executing the agreement later acquires the power of disposition.

Exchange Listing

Our common stock is listed on the Nasdaq Global Select Market under the symbol “FISI.”

Indemnification and Contribution

We have agreed to indemnify the underwriters and their affiliates, selling agents, and controlling persons against certain liabilities, including under the Securities Act. If we are unable to provide this indemnification, we will contribute to the payments the underwriters and their affiliates, selling agents, and controlling persons may be required to make in respect of those liabilities.

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Price Stabilization, Short Positions, and Penalty Bids

To facilitate this offering and in accordance with Regulation M under the Exchange Act, the underwriters may engage in transactions that stabilize, maintain, or otherwise affect the price of our common stock, including:

• stabilizing transactions;

• short sales; and

• purchase to cover positions created by short sales.

Stabilizing transactions consist of bids or purchases made for the purpose of preventing or retarding a decline in the market price of our common stock while this offering is in progress. These transactions may also include making short sales of our common stock, which involves the sale by the underwriters of a greater number of shares of common stock than they are required to purchase in this offering. Short sales may be “covered short sales,” which are short positions in an amount not greater than the underwriters’ purchase option referred to above, or may be “naked short sales,” which are short positions in excess of that amount.

The underwriters may close out any covered short position either by exercising their purchase option, in whole or in part, or by purchasing shares in the open market. In making this determination, the underwriters will consider, among other things, the price of shares available for purchase in the open market compared to the price at which they may purchase shares through the purchase option described above. The underwriters must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of our common stock in the open market that could adversely affect investors who purchased in this offering.

As an additional means of facilitating our public offering, the underwriters may bid for, and purchase, shares of our common stock in the open market. The underwriting syndicate also may reclaim selling concessions allowed to an underwriter or a dealer for distributing shares of our common stock in this offering, if the syndicate repurchases previously distributed shares of our common stock to cover syndicate short positions or to stabilize the price of our common stock.

As a result of these activities, the price of our common stock may be higher than the price that otherwise might exist in the open market. If these activities are commenced, they may be discontinued by the underwriters at any time without notice. The underwriters may carry out these transactions on the Nasdaq Global Select Market, in the over-the-counter market or otherwise.

Electronic Distribution

A prospectus supplement in electronic format may be made available by e-mail or on the websites or through online services maintained by one or more of the underwriters or their affiliates. In those cases, prospective investors may view offering terms online and may be allowed to place orders online. The underwriters may agree with us to allocate a specific number of shares for sale to online brokerage account holders. Any such allocation for online distributions will be made by the underwriters on the same basis as other allocations. Other than the prospectus supplement in electronic format, the information on the underwriters’ websites and any information contained on any other website maintained by any of the underwriters is not part of this prospectus supplement, has not been approved and/or endorsed by the underwriters or us, and should not be relied upon by investors.

Affiliations

The underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment advisory, investment research, principal investment, hedging, financing,

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loan referrals, valuation, and brokerage activities. From time to time, the underwriters and/or their respective affiliates have directly and indirectly engaged, and may in the future engage, in various financial advisory, investment banking loan referrals, and commercial banking services with us and our affiliates, for which they received or paid, or may receive or pay, customary compensation, fees, and expense reimbursement. In the ordinary course of their various business activities, the underwriters and their respective affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers, and those investment and securities activities may involve securities and/or instruments of ours. The underwriters and their respective affiliates may also make investment recommendations and/or publish or express independent research views in respect of those securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in those securities and instruments.

Selling Restrictions

Canada

The shares may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the shares must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 of National Instrument 33-105 Underwriting Conflicts (NI 33-105), the underwriters are not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

European Economic Area

In relation to each Member State of the European Economic Area, or Member State, no shares have been offered or will be offered pursuant to the offering to the public in that Member State prior to the publication of a prospectus in relation to the shares which has been approved by the competent authority in that Member State, all in accordance with the Prospectus Regulation, except that offers of shares may be made to the public in that Member State at any time under the following exemptions under the Prospectus Regulation:

• to any qualified investor as defined in the Prospectus Regulation;

• to fewer than 150 natural or legal persons (other than qualified investors as defined under the Prospectus Regulation), subject to obtaining the prior consent of the underwriters for any such offer;

• to investors who acquire shares for a total consideration of at least EUR 100,000 per investor, for each separate offer; or

• in any other circumstances falling within Article 1(4) of the Prospectus Regulation,

provided that no such offer of shares shall require the Company or any of the underwriters to publish a prospectus pursuant to Article 3 of the Prospectus Regulation or supplement a prospectus pursuant to Article 23 of the Prospectus Regulation.

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For the purposes of this provision, the expression an “offer to the public” in relation to any shares in any Member State means the communication in any form and by any means of sufficient information on the terms of the offer and any shares to be offered so as to enable an investor to decide to purchase or subscribe for any shares, and the expression “Prospectus Regulation” means Regulation (EU) 2017/1129.

United Kingdom

In relation to the United Kingdom, no shares have been offered or will be offered pursuant to the offering to the public in the United Kingdom prior to the publication of a prospectus in relation to the shares which has been approved by the Financial Conduct Authority, except that it may make an offer to the public in the United Kingdom of any shares at any time under the following exemptions under the UK Prospectus Regulation:

• to any legal entity which is a qualified investor as defined under Article 2 of the UK Prospectus Regulation;

• to fewer than 150 natural or legal persons (other than qualified investors as defined under Article 2 of the UK Prospectus Regulation), subject to obtaining the prior consent of the representatives for any such offer; or

• in any other circumstances falling within Section 86 of the FSMA,

provided that no such offer of the shares shall require the Company or any of the underwriters to publish a prospectus pursuant to Section 85 of the FSMA or supplement a prospectus pursuant to Article 23 of the UK Prospectus Regulation.

For the purposes of this provision, the expression an “offer to the public” in relation to the shares in the United Kingdom means the communication in any form and by any means of sufficient information on the terms of the offer and any shares to be offered so as to enable an investor to decide to purchase or subscribe for any shares and the expression “UK Prospectus Regulation” means the Prospectus Regulation as it forms part of domestic law by virtue of the European Union (Withdrawal) Act 2018.

In addition, in the United Kingdom, this document is being distributed only to, and is directed only at, and any offer subsequently made may only be directed at persons who are “qualified investors” (as defined in Article 2 of the UK Prospectus Regulation) (i) who have professional experience in matters relating to investments falling within Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended, or the Order, and/or (ii) who are high net worth companies (or persons to whom it may otherwise be lawfully communicated) falling within Article 49(2)(a) to (d) of the Order (all such persons together we refer to as “relevant persons”) or otherwise in circumstances which have not resulted and will not result in an offer to the public of the shares in the United Kingdom within the meaning of the FSMA. This prospectus supplement must not be acted on or relied on in the United Kingdom by persons who are not relevant persons. In the United Kingdom, any investment or investment activity, to which this prospectus supplement relates, is only available to, and will be engaged in with, relevant persons.

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LEGAL MATTERS

The validity of the shares of common stock offered hereby and certain other legal matters in connection with the offering will be passed upon for us by Luse Gorman, PC, Washington, D.C. Hogan Lovells US LLP, Washington, D.C., will pass upon certain legal matters for the underwriters.

EXPERTS

The consolidated financial statements of Financial Institutions, Inc. as of December 31, 2023 and 2022 and for each of the years in the three-year period ended December 31, 2023 and the effectiveness of internal control over financial reporting as of December 31, 2023 incorporated in this prospectus by reference from the Company’s Annual Report on Form 10-K for the year ended December 31, 2023 have been audited by RSM US LLP, an independent registered public accounting firm, as stated in their reports thereon, incorporated herein by reference, and have been incorporated into this prospectus and registration statement in reliance upon such reports and upon the authority of such firm as experts in accounting and auditing.

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PROSPECTUS

$200,000,000

Financial Institutions, Inc.

Debt Securities

Common Stock

Preferred Stock

Depositary Shares

Warrants

Purchase Contracts

Units

We may offer and sell from time to time up to $200,000,000 of unsecured debt securities, which may consist of: notes, debentures, or other evidences of indebtedness; shares of common stock; shares of preferred stock; depositary shares; warrants to purchase other securities; purchase contracts; and units consisting of any combination of the above securities. The debt securities and preferred stock may be convertible into or exchangeable for other securities of ours.

This prospectus provides you with a general description of the securities listed above. Each time we offer any securities pursuant to this prospectus, we will provide a prospectus supplement and, if necessary, a pricing supplement that will contain specific information about the terms of that offering. The prospectus supplement may also add, update or change information contained in this prospectus. This prospectus may not be used to offer or sell our securities without a prospectus supplement describing the method and terms of the offering. You should read this prospectus and any prospectus supplement together with the information described under the heading “Incorporation of Certain Information by Reference” before you make your investment decision.

Our common stock is traded on the Nasdaq under the symbol “FISI.” On November 20, 2024, the last reported per share sale price of our common stock was $26.38.

The securities offered by this prospectus or any prospectus supplement may be offered directly to investors or to or through underwriters, dealers or other agents. See “Plan of Distribution” in this prospectus. If any underwriters or dealers are involved in the sale of any securities offered by this prospectus and any accompanying prospectus supplement, their names, and any applicable purchase price, fee, commission or discount arrangement between or among them will be set forth, or will be calculable from the information set forth, in the accompanying prospectus supplement.

Investing in our common stock involves risk. You should carefully read the information included and incorporated by reference into this prospectus for a discussion of the factors you should carefully consider in determining whether to invest in our securities, including the discussion of risks described under “Risk Factors” on page 5 of this prospectus.

The offered securities are not deposits or obligations of a bank or savings association and are not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is December 4, 2024.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS	1
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS	1
FINANCIAL INSTITUTIONS, INC.	3
RISK FACTORS	5
WHERE YOU CAN FIND MORE INFORMATION	5
USE OF PROCEEDS	6
THE SECURITIES WE MAY OFFER	6
DESCRIPTION OF DEBT SECURITIES	7
DESCRIPTION OF COMMON STOCK	13
DESCRIPTION OF PREFERRED STOCK	16
DESCRIPTION OF DEPOSITARY SHARES	17
DESCRIPTION OF WARRANTS	18
DESCRIPTION OF PURCHASE CONTRACTS	20
DESCRIPTION OF UNITS	21
PLAN OF DISTRIBUTION	21
LEGAL MATTERS	23
EXPERTS	23

ABOUT THIS PROSPECTUS

This prospectus is part of a “shelf” registration statement on Form S-3 that we filed with the Securities and Exchange Commission (the “SEC”). Under the shelf registration process, we may from time to time offer and sell the debt securities, common stock, preferred stock, depositary shares, warrants, purchase contracts, or units consisting of a combination of any of the securities described in this prospectus in one or more offerings, up to a total dollar amount of $200,000,000. This prospectus provides a general description of the securities that we may offer and sell. Each time we offer these securities, we will provide a prospectus supplement and, if necessary, a pricing supplement, containing specific information about the terms of the offer. The prospectus supplement and any pricing supplement may add, update or change the information contained in this prospectus. If there is any inconsistency between the information in this prospectus and the applicable prospectus supplement, you should rely on the information in the prospectus supplement and any pricing supplement. Before investing in our securities, you should carefully review this prospectus, any prospectus supplement and any pricing supplement, together with the additional information described under the heading “Where You Can Find More Information.”

We have not authorized anyone to provide you with information in addition to or different from that contained in this prospectus, any accompanying prospectus supplement and any pricing supplement. We take no responsibility for, and can provide no assurances as to the reliability of, any information not contained in this prospectus, any accompanying prospectus supplement or any pricing supplement that we may authorize to be provided to you. This prospectus is an offer to sell only the securities offered hereby, but only under circumstances and in jurisdictions where it is lawful to do so. You should assume that the information in this prospectus, any accompanying prospectus supplement or any pricing supplement is accurate only as of the date on the front of the document (unless the information specifically indicates that another date applies) and that any information incorporated by reference is accurate only as of the date of the document incorporated by reference, regardless of the time of delivery of this prospectus, any accompanying prospectus supplement or any pricing supplement, or any sale of a security.

CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

Statements in this prospectus that are based on other than historical data are forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. Forward-looking statements provide current expectations or forecasts of future events and include, among others:

• statements with respect to the beliefs, plans, objectives, goals, guidelines, expectations, anticipations, and future financial condition, results of operations and performance of Financial Institutions, Inc. (the “Parent” or “FII”) and its direct and indirect subsidiaries (collectively, the “Company,” “we,” “our” or “us”); and

• statements preceded by, followed by or that include the words “may,” “could,” “should,” “would,” “believe,” “anticipate,” “estimate,” “expect,” “intend,” “plan”, “target,” “projects” or similar expressions.

These forward-looking statements are not guarantees of future performance, nor should they be relied upon as representing management’s views as of any subsequent date. Forward-looking statements involve significant risks and uncertainties and actual results may differ materially from those presented, either expressed or implied, in this prospectus, any prospectus supplement, and our Annual Report on Form 10-K for the fiscal year ended December 31, 2023 (the “Form 10-K”), which is incorporated by reference into this prospectus, including, but not limited to, those presented in the Management’s Discussion and Analysis of Financial Condition and Results of Operations. Factors that might cause such material differences include, but are not limited to:

• Fluctuations in market interest rates may affect our interest margins and income, demand for our products, defaults on loans, loan prepayments and the fair value of our financial instruments;

• Environmental, social and governance matters, and any related reporting obligations may impact our business;

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• If we experience greater credit losses than anticipated, earnings may be adversely impacted;

• We are subject to risks and losses resulting from fraudulent activities that could adversely impact our financial performance and results of operations;

• Geographic concentration in our loan portfolio may unfavorably impact our operations;

• Our commercial business and mortgage loans increase our exposure to credit risks;

• Our indirect and consumer lending involves risk elements in addition to normal credit risk;

• Lack of seasoning in portions of our loan portfolio could increase risk of credit defaults in the future;

• We accept deposits that do not have a fixed term, and which may be withdrawn by the customer at any time for any reason;

• We are subject to environmental liability risk associated with our lending activities;

• We operate in a highly competitive industry and market area;

• Legal and regulatory proceedings and related matters could adversely affect us and the banking industry in general;

• Any future FDIC insurance premium increases may adversely affect our earnings;

• We are highly regulated, and any adverse regulatory action may result in additional costs, loss of business opportunities, and reputational damage;

• We are subject to the CRA and fair lending laws, and failure to comply with these laws could lead to material penalties;

• The policies of the Federal Reserve have a significant impact on our earnings;

• Our investment advisory and wealth management operations are subject to risk related to the regulation of the financial services industry and market volatility;

• We make certain assumptions and estimates in preparing our financial statements that may prove to be incorrect, which could significantly impact the results of our operations, cash flows and financial condition, and we are subject to new or changing accounting rules and interpretations, and the failure by us to correctly interpret or apply these evolving rules and interpretations could have a material adverse effect;

• The value of our goodwill and other intangible assets may decline in the future;

• We may be unable to successfully implement our growth strategies, including the integration and successful management of newly acquired businesses;

• The introduction of new products and services may subject us to increased regulation and regulatory scrutiny and may affect our reputation;

• Acquisitions may disrupt our business and dilute shareholder value;

• Our tax strategies and the value of our deferred tax assets and liabilities could adversely affect our operating results and regulatory capital ratios;

• Liquidity is essential to our businesses;

• We rely on dividends from our subsidiaries for most of our revenue;

• If our risk management framework does not effectively identify or mitigate our risks, we could suffer losses;

• We face competition in staying current with technological changes and banking alternatives to compete and meet customer demands;

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• We rely on other companies to provide key components of our business infrastructure;

• A breach in security of our or third-party information systems, including the occurrence of a cyber incident or a deficiency in cybersecurity, or a failure by us to comply with New York State cybersecurity regulations, may subject us to liability, result in a loss of customer business or damage our brand image;

• The soundness of other financial institutions could adversely affect us;

• We may need to raise additional capital in the future and such capital may not be available on acceptable terms or at all;

• We may not pay or may reduce the dividends on our common stock;

• We may issue debt and equity securities or securities convertible into equity securities, any of which may be senior to our common stock as to distributions and in liquidation, which could dilute our current shareholders or negatively affect the value of our common stock;

• Our Certificate of Incorporation (as defined below), our Bylaws (as defined below), and certain banking laws may have an anti-takeover effect;

• The market price of our common stock may fluctuate significantly in response to a number of factors;

• We may not be able to attract and retain skilled people;

• We use financial models for business planning purposes that may not adequately predict future results;

• We depend on the accuracy and completeness of information about or from customers and counterparties;

• Our business may be adversely affected by conditions in the financial markets and economic conditions generally, including macroeconomic pressures such as inflation, supply chain issues, and geopolitical risks associated with international conflict; and

• Severe weather, natural disasters, public health emergencies and pandemics, acts of war or terrorism, and other external events could significantly impact our business.

We caution readers not to place undue reliance on any forward-looking statements, which speak only as of the date made, and advise readers that various factors, including those described above, could affect our financial performance and could cause our actual results or circumstances for future periods to differ materially from those anticipated or projected. See also Item 1A, Risk Factors, in the Form 10-K, which is incorporated herein by reference, for further information. Except as required by law, we do not undertake, and specifically disclaim any obligation to publicly release any revisions to any forward-looking statements to reflect the occurrence of anticipated or unanticipated events or circumstances after the date of such statements.

FINANCIAL INSTITUTIONS, INC.

Financial Institutions, Inc. (the “Parent”) is a financial holding company organized in 1931 under the laws of New York State (“New York” or “NYS”). The principal office of the Parent is located at 220 Liberty Street, Warsaw, New York 14569 and its telephone number is (585) 786-1100. The Parent was incorporated on September 15, 1931, but the continuity of the Company’s banking business is traced to the organization of the National Bank of Geneva on March 28, 1817. Except as the context otherwise requires, the Parent and its direct and indirect subsidiaries are collectively referred to in this report as the “Company.” The Parent’s common stock is traded on the Nasdaq Global Select Market under the ticker symbol “FISI.” Five Star Bank is referred to as “FSB” or “the Bank,” and Courier Capital, LLC is referred to as “Courier Capital.”

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At September 30, 2024, the Company had consolidated total assets of $6.16 billion, deposits of $5.31 billion and shareholders’ equity of $500.3 million.

The Parent’s primary business is the operation of its subsidiaries. It does not engage in any other substantial business activities. The Parent’s two direct wholly-owned subsidiaries are: (1) the Bank, which provides a full range of banking services to consumer, commercial and municipal customers in Western and Central New York and commercial loans in the Mid-Atlantic and Central New York regions; and (2) Courier Capital, which provides customized investment management, investment consulting and retirement plan services to individuals, businesses, institutions, foundations and retirement plans.

Our business strategy has been to maintain a community bank philosophy, which consists of focusing on and understanding the individualized banking and other financial services needs of individuals, municipalities and businesses of the communities surrounding our primary service area. We believe this focus allows us to be more responsive to our customers’ needs and provide a high level of personal service that differentiates us from larger competitors, resulting in long-standing and broad-based banking relationships. Our core customers are primarily small- to medium-sized businesses, individuals and community organizations who prefer to build banking and wealth management relationships with a community bank that offers high-quality, competitively-priced products and services with personalized service. Because of our identity and origin as a locally operated bank, we believe that our level of personal service provides a competitive advantage over larger banks, which tend to consolidate decision-making authority outside local communities.

A key aspect of our current business strategy is to foster a community–oriented culture where our customers and employees establish long-standing and mutually beneficial relationships. We believe that we are well-positioned to be a strong competitor within our market area because of our focus on community banking needs and customer service, our comprehensive suite of deposit, loan, insurance and wealth management products and services typically found at larger banks, our highly experienced management team and our strategically located banking centers.

We prioritize customer acquisition through cost-effective, high-demand digital, virtual and physical channels, while maintaining a community bank distinctiveness relative to larger banks and digital-only neobanks. We leverage the retail branch network and customer contact center to build trust and credibility, provide personal financial education and advice, offer convenience, and bridge digital and physical channels. Our enhanced digital capabilities complement a continued focus on a consistent customer experience and engagement across physical and virtual channels, including using branches to create deeper engagement and relationships with customers, balancing customer engagement with efficiency opportunities (e.g., framing outreach to the customer contact center to teach customers how to use digital channels, in addition to addressing the reason for the call), and maintaining and expanding our customer reach digitally, physically or virtually. By employing digital channels across our current products and services, we deepen existing relationships and enter new geographies or market segments that would otherwise be prohibitively expensive targets using traditional approaches. Deepening our existing digital capabilities allows us to capitalize on a shift in customer preferences away from physical branches.

We have evolved to meet changing customer needs by offering complementary physical, digital and virtual channels. We focus on technology to provide solutions that fit our customers’ preferences for transacting business with us. Branches are staffed by certified personal bankers who are trained to meet a broad array of customer needs. Our digital banking capabilities, interactive teller machine (“ITM”) functionality and Customer Contact Center provide additional self-serve and phone options through which customer needs are met effectively.

We will continue to explore market expansion opportunities that complement current market areas as opportunities arise. Our primary focus will be on increasing the Bank’s market share within existing markets, while taking advantage of potential growth opportunities within our noninterest income lines of business by

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acquiring businesses that can be incorporated into existing operations. We believe our capital position remains strong enough to support an active merger and acquisition strategy and the expansion of our core financial service businesses. Consequently, we continue to explore acquisition opportunities in these activities. When evaluating acquisition opportunities, we will balance the potential for earnings accretion with maintaining adequate capital levels, which could result in our common stock being the predominant form of consideration and/or the need for us to raise capital.

Conversations with potential strategic partners occur on a regular basis. The evaluation of any potential opportunity will favor a transaction that complements our core competencies and strategic intent, with a lesser emphasis being placed on geographic location or size. Additionally, we remain committed to maintaining a diversified revenue stream. Our senior management team has experience in acquisitions and post-acquisition integration of operations and is prepared to act promptly should a potential opportunity arise but will remain disciplined with its approach. We believe this experience positions us to successfully acquire and integrate additional financial services and banking businesses.

RISK FACTORS

Investing in our securities involves risks. Before making an investment decision, you should carefully consider the specific risks set forth under the caption “Risk Factors” in the applicable prospectus supplement and under the caption “Risk Factors” in our filings with the SEC, which are incorporated by reference into this prospectus.

WHERE YOU CAN FIND MORE INFORMATION

The SEC maintains an internet site that contains reports, proxy and information statements, and other information regarding issuers that file electronically with the SEC. We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public at the SEC’s web site at www.sec.gov.

The SEC allows us to “incorporate by reference” into this prospectus the information in documents we file with the SEC, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be a part of this prospectus and should be read with the same care. When we update the information contained in documents that have been incorporated by reference, by making future filings with the SEC, the information incorporated by reference in this prospectus is considered to be automatically updated and superseded. In other words, in all cases, if you are considering whether to rely on information contained in this prospectus or information incorporated by reference into this prospectus, you should rely on the information contained in the document that was filed later. We incorporate by reference the documents listed below (File No. 000-26481 except as otherwise indicated), which are considered to be a part of this prospectus (in each case, excluding any information “furnished” to, rather than filed with, the SEC, including, but not limited to, information furnished under Items 2.02 or 7.01 of Form 8-K and any corresponding information furnished with respect to such Items under Item 9.01 or as an exhibit):

• our Annual Report on Form 10-K for the year ended December 31, 2023, filed with the SEC on March 13, 2024;

• the portions of our Definitive Proxy Statement on  Schedule 14A, filed with the SEC on April 12, 2024 that are incorporated by reference in our Annual Report on Form 10-K for the year ended December 31, 2023;

• our Quarterly Reports on Form 10-Q for the quarter ended March 31, 2024, filed with the SEC on May 6, 2024, for the quarter ended June  30, 2024, filed with the SEC on August 5, 2024, and for the quarter ended September 30, 2024, filed with the SEC on November 4, 2024;

• our Current Reports on Form 8-K filed with the SEC on March 14, 2024, March  27, 2024, April  1, 2024, June  6, 2024, June  25, 2024, and September 16, 2024; and

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• The description of our common stock set forth in the registration statement on  Form 8-A, filed with the SEC on June 23, 1999, to register our common stock under the Exchange Act, as updated and amended from time to time.

All reports and other documents we subsequently file under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, prior to the termination of this offering, including all such documents we may file with the SEC after the date of the initial registration statement and prior to the effectiveness of the registration statement, but excluding any information furnished to, rather than filed with, the SEC, will also be incorporated by reference into this prospectus and deemed to be part of this prospectus from the date of the filing of such reports and documents. The most recent information that we file with the SEC automatically updates and supersedes older information. The information contained in any such filing will be deemed to be a part of this prospectus, commencing on the date on which the document is filed.

Any documents incorporated by reference into this prospectus are available without charge to you on the Internet at www.FISI-investors.com under the “Financials” page or if you call or write to: Samuel J. Burruano Jr., Corporate Secretary, Financial Institutions, Inc., 220 Liberty Street, Warsaw, New York 14569, telephone: (585) 786-1100. The reference to our website is not intended to be an active link and the information on our website is not, and you must not consider the information to be, a part of this prospectus.

You should rely only on the information contained or incorporated by reference in this prospectus and the applicable prospectus supplement. Neither we nor any underwriter or agent have authorized anyone else to provide you with additional or different information. We may only use this prospectus to sell securities if it is accompanied by a prospectus supplement. We are only offering these securities in jurisdictions where the offer is permitted. You should not assume that the information in this prospectus or the applicable prospectus supplement or any document incorporated by reference is accurate as of any date other than the dates of the applicable documents.

USE OF PROCEEDS

The Company intends to use the net proceeds from the sale of any securities offered under this prospectus in the manner and for the purposes set forth in the applicable prospectus supplement, which may include general corporate purposes.

THE SECURITIES WE MAY OFFER

This prospectus contains a summary of the debt securities, common stock, preferred stock, depositary shares, warrants, purchase contracts, and units that we may offer under this prospectus. The particular material terms of the securities offered by a prospectus supplement will be described in that prospectus supplement. The descriptions herein and in the applicable prospectus supplement do not contain all of the information that you may find useful or that may be important to you. However, this prospectus, the prospectus supplement and the pricing supplement, if applicable, contain the material terms and conditions for each security. The prospectus supplement will also contain information, where applicable, about material U.S. federal income tax considerations relating to the offered securities, and the securities exchange, if any, on which the offered securities will be listed. You should read these documents as well as the documents filed as exhibits to or incorporated by reference to this registration statement. Capitalized terms used in this prospectus that are not defined will have the meanings given them in these documents.

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DESCRIPTION OF DEBT SECURITIES

General

We may issue senior debt securities or subordinated debt securities. Senior debt securities will be issued under an indenture, referred to as the “senior indenture,” and subordinated debt securities will be issued under a separate indenture, referred to in this section as the “subordinated indenture.” The senior indenture and the subordinated indenture are collectively referred to in this section as the “indentures.” The senior debt securities and the subordinated debt securities are collectively referred to in this section as the “debt securities.” The debt securities will be our direct unsecured general obligations.

This prospectus describes the general terms and provisions of the debt securities. When we offer to sell a particular series of debt securities, we will describe the specific terms of the securities in a supplement to this prospectus. The prospectus supplement will also indicate whether the general terms and provisions described in this prospectus apply to a particular series of debt securities.

The following briefly describes the general terms and provisions of the debt securities and the indentures. We have not restated these indentures in their entirety in this description. We have filed the forms of the indentures, including the forms of debt securities, as exhibits to the registration statement of which this prospectus is a part. We urge you to read the indentures, because they, and not this description, control your rights as holders of the debt securities. The following description of the indentures is not complete and is subject to, and qualified in its entirety by reference to, all the provisions in the respective indentures. Capitalized terms used in the summary have the meanings specified in the indentures.

Neither indenture limits the amount of debt securities that we may issue under the indenture from time to time in one or more series. We may in the future issue debt securities under either indenture. As of the date of this prospectus, we had not issued any debt securities under either indenture.

Neither indenture contains provisions that would afford holders of debt securities protection in the event of a sudden and significant decline in our credit quality or a takeover, recapitalization or highly leveraged or similar transaction. Accordingly, we could in the future enter into transactions that could increase the amount of indebtedness outstanding at that time or otherwise adversely affect our capital structure or credit rating.

The debt securities will be exclusively our obligations. The indentures do not require our subsidiaries to guarantee the debt securities. As a result, the holders of debt securities will generally have a junior position to claims of all creditors and preferred shareholders of our subsidiaries.

Terms of Each Series of Debt Securities Provided in the Prospectus Supplement

A prospectus supplement and any supplemental indenture relating to any series of debt securities being offered will include specific terms relating to the offering. These terms will include some or all of the following:

• the form and title of the debt securities;

• whether the debt securities are senior debt securities or subordinated debt securities and the terms of subordination;

• the principal amount of the debt securities;

• the denominations in which the debt securities will be issued;

• the portion of the principal amount which will be payable if the maturity of the debt securities is accelerated;

• the currency or currency unit in which the debt securities will be paid, if not U.S. dollars;

• any right we may have to defer payments of interest by extending the dates payments are due and whether interest on those deferred amounts will be payable as well;

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• the place where the principal of, and premium, if any, and interest on any debt securities will be payable;

• the date or dates on which the debt securities will be issued and the principal, and premium, if any, of the debt securities will be payable;

• the rate or rates which the debt securities will bear interest and the interest payment dates for the debt securities;

• any mandatory or optional redemption provisions;

• the terms, if any, upon which the debt securities are convertible into other securities of ours or another issuer and the terms and conditions upon which any conversion will be effected, including the initial conversion price or rate, the conversion period and any other provisions in addition to or instead of those described in this prospectus;

• any sinking fund or other provisions that would obligate us to repurchase or otherwise redeem the debt securities;

• any deletion from, changes of or additions to the covenants or the Events of Default (as defined below) under “Provisions in Both Indentures — Events of Default and Remedies”;

• any changes to the terms and conditions upon which the debt securities can be defeased or discharged;

• any restriction or other provision with respect to the transfer or exchange of the debt securities;

• the identity of any other trustee, paying agent and security registrar, if other than the trustee; and

• any other terms of the debt securities.

We will maintain in each place specified by us for payment of any series of debt securities an office or agency where debt securities of that series may be presented or surrendered for payment, where debt securities of that series may be surrendered for registration of transfer or exchange and where notices and demands to or upon us in respect of the debt securities of that series and the related indenture may be served.

Debt securities may be issued under an indenture as original issue discount securities to be offered and sold at a substantial discount below their principal amount. Material federal income tax, accounting and other considerations applicable to any such original issue discount securities will be described in any related prospectus supplement. “Original issue discount security” means any security which provides for an amount less than the principal amount thereof to be due and payable upon a declaration of acceleration of the maturity thereof as a result of the occurrence of an Event of Default and the continuation thereof.

Provisions Only in the Senior Indenture

Payment of the principal, premium, if any, and interest on the unsecured senior debt securities will rank equally in right of payment with all of our other unsecured senior debt.

Provisions Only in the Subordinated Indenture

Payment of the principal, premium, if any, and interest on the subordinated debt securities will be unsecured and will be subordinate and junior in priority of payment to prior payment in full of all of our senior indebtedness, including senior debt securities and other debt to the extent described in a prospectus supplement.

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Provisions in Both Indentures

Consolidation, Merger or Asset Sale

Each indenture generally allows us to consolidate or merge with a domestic person, association or entity. They also allow us to sell, lease or transfer our property and assets substantially as an entirety to a domestic person, association or entity. If this happens, the remaining or acquiring person, association or entity must assume all of our responsibilities and liabilities under the indentures including the payment of all amounts due on the debt securities and performance of the covenants in the indentures.

However, we will only consolidate or merge with or into any other person, association or entity, or sell, lease or transfer our assets substantially as an entirety according to the terms and conditions of the indentures, which require that:

• the remaining or acquiring person, association or entity is organized under the laws of the United States, any state within the United States or the District of Columbia;

• the remaining or acquiring person, association or entity assumes our obligations under the indentures; and

• immediately after giving effect to the transaction, no Default or Event of Default, as defined below, shall have occurred and be continuing.

The remaining or acquiring person, association or entity will be substituted for us in the indentures with the same effect as if it had been an original party to the indentures. Thereafter, the successor may exercise our rights and powers under the indentures, in our name or in its own name. If we sell or transfer all or substantially all of our assets, we will be released from all our liabilities and obligations under any indenture and under the debt securities. If we lease all or substantially all of our assets, we will not be released from our obligations under the indentures.

Events of Default and Remedies

Pursuant to the indentures, “Default” with respect to any series of debt securities means any event which is an Event of Default, or any event that, after notice is given or lapse of time or both, would become an Event of Default. The term “Event of Default” with respect to any series of debt securities means any of the following:

• failure to pay the principal of or any premium on any debt security of that series when due;

• failure to pay interest on any debt security of that series for 30 days;

• subject to certain exceptions, failure to perform any other covenant in the indenture, other than a covenant default in the performance of which has expressly been included in the indenture solely for the benefit of series of debt securities other than that series, that continues for 90 days after being given written notice as specified in the indenture;

• our bankruptcy, insolvency or reorganization; or

• any other Event of Default included in any indenture or supplemental indenture.

If an Event of Default with respect to a series of debt securities occurs and is continuing, the trustee or the holders of at least 25% in principal amount of all of the outstanding debt securities of a particular series may declare the principal of all the debt securities of that series to be due and payable. When such declaration is made, such amounts will be immediately due and payable. The holders of a majority in principal amount of the outstanding debt securities of such series may rescind such declaration and its consequences if all existing Events of Default have been cured or waived, other than nonpayment of principal or interest that has become due solely as a result of acceleration. Upon a bankruptcy event of us, the principal amount of all debt securities shall be immediately due and payable.

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Holders of a series of debt securities may not enforce the indenture or the series of debt securities, except as provided in the indenture or a series of debt securities. The trustee may require indemnity satisfactory to it before it enforces the indenture or such series of debt securities. Subject to certain limitations, the holders of a majority in principal amount of the outstanding debt securities of a particular series may direct the time, method and place of conducting any proceeding for any remedy available to the trustee or exercising any trust or power of the trustee. The trustee may withhold notice to the holders of debt securities of any default, except in the payment of principal or interest, if it considers such withholding of notice to be in the best interests of the holders.

An Event of Default for a particular series of debt securities does not necessarily constitute an Event of Default for any other series of debt securities issued under an indenture. Further, an Event of Default under the debt securities of any series will not necessarily constitute an event of default under our other indebtedness or vice versa.

Modification of Indentures

Under each indenture, generally we and the trustee may modify our rights and obligations and the rights of the holders with the consent of the holders of a majority in aggregate principal amount of the outstanding debt securities of any series affected by the modification, voting as one class. No modification of the principal or interest payment terms, no modification reducing the percentage required for modifications and no modification impairing the right to institute suit for the payment on debt securities of any series when due, is effective against any holder without its consent.

In addition, we and the trustee may amend the indentures without the consent of any holder of the debt securities to make certain technical changes, such as:

• curing ambiguities or correcting defects or inconsistencies;

• evidencing the succession of another person to us, and the assumption by that successor of our obligations under the applicable indenture and the debt securities of any series;

• providing for a successor trustee;

• qualifying the indentures under the Trust Indenture Act of 1939, as amended, which we refer to in this prospectus as the “Trust Indenture Act”;

• complying with the rules and regulations of any securities exchange or automated quotation system on which debt securities of any series may be listed or traded; or

• adding provisions relating to a particular series of debt securities.

Discharging Our Obligations

We may choose either to discharge our obligations on the debt securities of any series in a legal defeasance, or to release ourselves from our covenant restrictions on the debt securities of any series in a covenant defeasance. We may do so at any time on the 91st day after we deposit with the trustee sufficient cash or government securities to pay the principal, interest, any premium and any other sums due to the stated maturity date or a redemption date of the debt securities of the series. If we choose the legal defeasance option, the holders of the debt securities of the series will not be entitled to the benefits of the indenture except for registration of transfer and exchange of debt securities, replacement of lost, stolen or mutilated debt securities, conversion or exchange of debt securities, sinking fund payments and receipt of principal and interest on the original stated due dates or specified redemption dates.

We may discharge our obligations on the debt securities of any series or release ourselves from covenant restrictions only if we meet certain requirements. Among other things, we must deliver an opinion of our legal counsel that the discharge will not result in holders having to recognize taxable income or loss or subject them to

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different tax treatment. In the case of legal defeasance, this opinion must be based on either an IRS letter ruling or change in federal tax law. We may not have a default on the debt securities discharged on the date of deposit. The discharge may not violate any of our agreements. The discharge may not result in our becoming an investment company in violation of the Investment Company Act of 1940.

Information Concerning the Indenture Trustee

We will designate a trustee under the senior indenture and the subordinated indenture before the issuance of the debt securities.

Under provisions of the indentures and the Trust Indenture Act governing trustee conflicts of interest, any uncured Event of Default with respect to any series of senior or subordinated debt securities will force the trustee to resign as trustee under either the subordinated indenture or the senior indenture. Any resignation will require the appointment of a successor trustee under the applicable indenture in accordance with its terms and conditions.

The trustee may resign or be removed by us with respect to one or more series of debt securities and a successor trustee may be appointed to act with respect to any such series. The holders of a majority in aggregate principal amount of the debt securities of any series may remove the trustee with respect to the debt securities of such series.

Each indenture contains certain limitations on the right of the trustee thereunder, if it becomes our creditor, to obtain payment of claims in some cases, or to realize on property received in respect of any such claim, as security or otherwise.

The trustee is required to submit an annual report to the holders of the debt securities regarding, among other things, the trustee’s eligibility to serve, the priority of the trustee’s claims regarding certain advances made by it, and any action taken by the trustee materially affecting the debt securities.

Each indenture provides that, in addition to other certificates or opinions that may be specifically required by other provisions of an indenture, every application by us for action by the trustee shall be accompanied by a certificate of our officers and an opinion of counsel, who may be our counsel, stating that, in the opinion of the signers, we have complied with all conditions precedent to the action.

Payment and Paying Agent

Unless we otherwise indicate in the accompanying prospectus supplement, we will make payment of the interest on any debt securities on any interest payment date to the person in whose name the debt securities, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest.

We will pay principal of and any premium and interest on the debt securities of a particular series at the office of the paying agents designated by us, except that, unless we otherwise indicate in the accompanying prospectus supplement, we may make interest payments by check which we will mail to the holder or by wire transfer to certain holders. Unless we otherwise indicate in a prospectus supplement, we will designate an office or agency of the trustee in the contiguous United States as our sole paying agent for payments with respect to debt securities of each series. We will name in the accompanying prospectus supplement any other paying agents that we initially designate for the debt securities of a particular series. We will maintain a paying agent in each place of payment for the debt securities of a particular series.

Governing Law

The indentures are and the debt securities will be governed by the laws of the State of New York.

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No Personal Liability of Officers, Directors, Employees or Shareholders

Our officers, directors, employees and shareholders will not have any liability for our obligations under the indentures or the debt securities by way of his or her status. Each holder of debt securities, by accepting a debt security, waives and releases all such liability. The waiver and release are part of the consideration for the issuance of the debt securities.

Form, Denominations and Registration; Global Securities; Book Entry Only System

Unless otherwise indicated in a prospectus supplement, the debt securities of a series will be issued only in fully registered form, without coupons, in minimum denominations of $1,000 or integral multiples in excess thereof. You will not have to pay a service charge to transfer or exchange debt securities of a series, but we may require you to pay for taxes or other governmental charges due upon a transfer or exchange.

Unless otherwise indicated in a prospectus supplement, each series of debt securities will be deposited with, or on behalf of, The Depository Trust Company, or DTC, or any successor depositary, which we call a “depositary”, and will be represented by one or more global notes registered in the name of Cede & Co., as nominee of DTC. The interests of beneficial owners in the global notes will be represented through financial institutions acting on their behalf as direct or indirect participants in DTC.

Ownership of beneficial interests in a global note will be limited to persons, called participants, who have accounts with DTC or persons who hold interests through participants. Ownership of beneficial interests in the global notes will be shown on, and the transfer of these ownership interests will be effected only through, records maintained by DTC or its nominee (with respect to interests of participants) and the records of participants (with respect to interests of persons other than participants).

So long as DTC, or its nominee, is the registered owner or holder of a global note, DTC or such nominee, as the case may be, will be considered the sole owner or holder of the debt securities of that series represented by such global note for all purposes of the indenture, the debt securities of that series and applicable law. In addition, no beneficial owner of an interest in a global note will be able to transfer that interest except in accordance with DTC’s applicable procedures, in addition to those under the applicable indenture.

Payments on debt securities represented by global notes will be made to DTC or its nominee, as the registered owner thereof. Neither we, the trustee, any underwriter nor any paying agent will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in global notes, for maintaining, supervising or reviewing any records relating to such beneficial ownership interests or for any action taken or omitted to be taken by the depositary or any participant.

We expect that DTC or its nominee will credit participants’ accounts on the payable date with payments in respect of a global note in amounts proportionate to their respective beneficial interest in the principal amount of such global note as shown on the records of DTC or its nominee, unless DTC has reason to believe that it will not receive payment on the payable date. We also expect that payments by participants to owners of beneficial interests in such global note held through such participants will be governed by standing instructions and customary practices, as is now the case with securities held for the accounts of customers registered in “street name.” Such payments will be the responsibility of such participants.

Transfers between participants in DTC will be effected in accordance with DTC rules. The laws of some states require that certain persons take physical delivery of securities in definitive form. Consequently, the ability to transfer beneficial interests in a global note to such persons may be impaired. Because DTC can only act on behalf of participants, who in turn act on behalf of others, such as securities brokers and dealers, banks and trust companies, called indirect participants, the ability of a person having a beneficial interest in a global note to pledge that interest to persons or entities that do not participate in the DTC system, or otherwise take actions in respect of that interest, may be impaired by the lack of a physical certificate of that interest.

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DTC will take any action permitted to be taken by a holder of debt securities of a series only at the direction of one or more participants to whose account interests in global notes are credited and only in respect of such portion of the aggregate principal amount of the debt securities of a series as to which such participant or participants has or have given such direction.

If: (1) the depositary notifies us that it is unwilling or unable to continue as depositary or if the depositary ceases to be eligible under the applicable indenture and a successor depositary is not appointed by us within 90 days; or (2) an Event of Default with respect to a series of debt securities shall have occurred and be continuing, the respective global notes representing the affected series of debt securities will be exchanged for debt securities in definitive form of like tenor and of an equal aggregate principal amount, in authorized denominations. Such definitive debt securities will be registered in such name or names as the depositary instructs the trustee. Such instructions will most likely be based upon directions received by the depositary from participants with respect to ownership of beneficial interests in global notes.

DTC is a limited-purpose trust company organized under the New York Banking Law, a “banking organization” within the meaning of the New York Banking Law, a member of the Federal Reserve, a “clearing corporation” within the meaning of the New York Uniform Commercial Code and a “clearing agency” registered pursuant to the provisions of Section 17A of the Exchange Act. DTC holds securities that its participants deposit with DTC and facilitates the settlement among participants of securities transactions, such as transfers and pledges, in deposited securities through electronic computerized book-entry changes in participants’ accounts, thereby eliminating the need for physical movement of securities certificates. Direct participants include securities brokers and dealers, banks, trust companies, clearing corporations and certain other organizations. DTC is owned by a number of its direct participants, including those who may act as underwriters of our debt securities, and by the New York Stock Exchange, Inc., the NYSE American and the Financial Industry Regulatory Authority. Access to the DTC system is also available to others such as indirect participants that clear through or maintain a custodial relationship with a direct participant, either directly or indirectly. The rules applicable to DTC and its participants are on file with the SEC.

Although DTC has agreed to the foregoing procedures to facilitate transfers of interests in global notes among participants of DTC, it is under no obligation to perform or continue to perform such procedures, and such procedures may be discontinued at any time. Neither we, the trustee, any underwriter nor any paying agent will have any responsibility for the performance by DTC or its participants or indirect participants of their respective obligations under the rules and procedures governing their operations.

DESCRIPTION OF COMMON STOCK

Introduction

The following section describes the material features and rights of our common stock. The summary does not purport to be exhaustive and is qualified in its entirety by reference to our Amended and Restated Certificate of Incorporation (the “Certificate of Incorporation”) and our Amended and Restated Bylaws (the “Bylaws”), each of which is filed as an exhibit to the Registration Statement of which this prospectus is a part, and to applicable sections of the Business Corporation Law of the State of New York (the “NYBCL”).

General

We are authorized to issue up to 50,000,000 shares of common stock. At November 20, 2024, 15,474,634 shares of common stock were outstanding. Each share of common stock entitles the holder to the same rights, and is the same in all respects, as each other share of common stock. Holders of common stock are entitled to: (1) one vote per share on all matters requiring a shareholder vote; (2) a ratable distribution of dividends on the common stock, if and when, declared by the board of directors (the “Board”); and (3) in the event of a liquidation, dissolution or winding up of us, to share ratably in all assets remaining after holders of shares of preferred stock have received

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the liquidation preference of their shares plus accumulated but unpaid dividends (whether or not earned or declared), if any, and after all of our other indebtedness has been provided for or satisfied. Holders of common stock do not have cumulative voting rights with respect to the election of directors and have no preemptive rights to acquire any of our additional, unissued or treasury shares or our securities convertible into or carrying a right to subscribe for or acquire our shares of capital stock. The shares of common stock, when issued in the manner described in this prospectus, will be fully paid and nonassessable.

Dividends

We have historically paid regular quarterly cash dividends on our common stock and the Board presently intends to continue the payment of regular quarterly cash dividends, dependent upon our results of operations, tax considerations, capital requirements and subject to the need for those funds for debt service and other purposes and the restrictions described below. The amount of future dividends will depend upon earnings, financial condition, capital requirements, other regulatory requirements and other factors, and will be determined by our Board on a quarterly basis. We cannot guarantee that we will continue to pay dividends or that, if paid, we will not reduce or eliminate dividends in the future.

New York State law applicable to the declaration of distributions by a business corporation also limits our ability to declare and pay dividends. A corporation generally may not authorize and make distributions if, after giving effect thereto, it would be unable to meet its debts as they become due in the usual course of business or if the corporation’s total assets would be less than the sum of its total liabilities plus the amount that would be needed, if it were to be dissolved at the time of distribution, to satisfy claims upon dissolution of shareholders who have rights superior to the rights of the holders of its common stock.

In addition, as a bank holding company, our ability to declare and pay dividends is subject to the guidelines of the Federal Reserve Board regarding capital adequacy and dividends. It is the policy of the Federal Reserve Board that bank holding companies should pay cash dividends on common stock only out of income available over the past year, and only if prospective earnings retention is consistent with the holding company’s expected future needs and financial condition. The policy provides that bank holding companies should not maintain a level of cash dividends that undermines the bank holding company’s ability to serve as a source of strength to its subsidiaries.

While we are a legal entity separate and distinct from our banking and other subsidiaries, these subsidiaries are the principal assets, and as such, a substantial part of our operating funds and, for the foreseeable future, all of the funds available for the payment of dividends are derived from the Bank and our other subsidiaries. The New York Banking Law provides that the Bank may not declare dividends during the calendar year that exceed the sum of the Bank’s net income during the current calendar year and the retained net income of the prior two calendar years unless the dividend has been approved by the Superintendent of Banks of the State of New York. Thus, future dividends will depend upon the earnings of the Bank, its financial condition and its need for funds. Capital adequacy requirements serve to limit the amount of dividends that may be paid by the Bank. Under federal law, the Bank cannot pay a dividend if, after paying the dividend, the Bank will be “undercapitalized.” The FDIC may declare a dividend payment to be unsafe and unsound even though the Bank would continue to meet its capital requirements after the dividend. Also, because the Company is a legal entity separate and distinct from its subsidiaries, our right to participate in the distribution of assets of any subsidiary upon the subsidiary’s liquidation or reorganization will be subject to the prior claims of the subsidiary’s creditors. In the event of a liquidation or other resolution of an insured depository institution, the claims of depositors and other general or subordinated creditors are entitled to a priority of payment over the claims of holders of any obligation of the institution to its shareholders, including any depository bank holding company (such as the Company) or any shareholder or creditor thereof.

Holders of common stock are entitled to participate equally in dividends or other distributions when, as and if declared by the Board out of funds legally available therefor. Subject to those regulatory restrictions described

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above, dividends may be paid in cash, property or common shares, unless we are insolvent or the dividend payment would render it insolvent.

Transfer Agent & Registrar

The transfer agent and registrar for our common stock is Equiniti Trust Company, LLC.

Anti-Takeover Effects of Certain Provisions in our Certificate of Incorporation, Bylaws and the NYBCL

Some provisions of our Certificate of Incorporation, our Bylaws, and the NYBCL may be deemed to have an anti-takeover effect and may collectively operate to delay, defer or prevent a tender offer, a proxy contest or takeover attempt that a shareholder might consider in his or her best interest, including those attempts that might result in a premium over the market price for the shares held by our shareholders. These provisions are intended to discourage certain types of coercive takeover practices and inadequate takeover bids. This also encourages persons seeking to acquire control of us to negotiate with us first. As a result, shareholders who might desire to participate in such transactions may not have an opportunity to do so. In addition, these provisions will also render the removal of our Board or management more difficult. The following discussion is a summary of certain material provisions of our Certificate of Incorporation and our Bylaws, copies of which are filed as exhibits to the Registration Statement of which this prospectus is a part.

Directors. The Board is divided into three classes. The members of each class are elected for a term of three years and only one class of directors is elected annually. Thus, it would take at least two annual elections to replace a majority of our Board. Further, the Bylaws impose notice and information requirements in connection with the nomination by shareholders of candidates for election to the Board or a proposal by shareholders of business to be acted upon at an annual meeting of shareholders.

Advance Notice of Shareholder Proposals and Nominations. Our Bylaws include an advance notice procedure for shareholders to nominate candidates for election as directors or to bring other business before any meeting of our shareholders. The shareholder notice procedure provides that only persons who are nominated by, or at the direction of, the Board, or by a shareholder who has given timely written notice prior to the meeting at which directors are to be elected, will be eligible for election as directors and that, at a shareholders’ meeting, only such business may be conducted as has been brought before the meeting by, or at the direction of, the Board or by a shareholder who has given timely written notice of such shareholder’s intention to bring such business before such meeting.

Under the shareholder notice procedure, for notice of shareholder nominations or other business to be made at a shareholders’ meeting to be timely, such notice must be received by us not earlier than the close of business on the 120^th calendar day and not later than the close of business on the 90^th calendar day prior to the one-year anniversary of the immediately preceding year’s annual meeting or as otherwise provided in the Bylaws.

A shareholder’s notice to us proposing to nominate a person for election as a director or proposing other business must contain certain information specified in the Bylaws, including the identity and address of the nominating shareholder, a representation that the shareholder is a record holder of our stock entitled to vote at the meeting and information regarding each proposed nominee or each proposed matter of business that would be required under the federal securities laws to be included in a proxy statement soliciting proxies for the proposed nominee or the proposed matter of business.

The shareholder notice procedure may have the effect of precluding a contest for the election of directors or the consideration of shareholder proposals if the proper procedures are not followed, and of discouraging or deterring a third party from conducting a solicitation of proxies to elect its own slate of directors or to approve its own proposal, without regard to whether consideration of such nominees or proposals might be harmful or beneficial to us and our shareholders.

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Restrictions on Call of Special Meetings. Our Bylaws provide that special meetings of shareholders can only be called by the Board, the Chairman of the Board or by the Secretary of the Company upon the written request of the holders of at least a majority of the outstanding shares entitled to vote at the meeting.

Prohibition of Cumulative Voting. The Certificate of Incorporation does not authorize cumulative voting for the election of directors.

Preferred Stock Authorization. Our Board, without shareholder approval, has the authority under our Certificate of Incorporation to issue preferred stock with rights superior to the rights of the holders of common stock. As a result, preferred stock, while not intended as a defensive measure against takeovers, could be issued quickly and easily, could adversely affect the rights of holders of common stock and could be issued with terms calculated to delay or prevent a change of control of the Company or make removal of management more difficult.

DESCRIPTION OF PREFERRED STOCK

We are authorized to issue up to 210,000 shares of two classes of preferred stock, consisting of: 10,000 shares of Class A Preferred Stock and 200,000 shares of Class B Preferred Stock. The Certificate of Incorporation provides that both classes of Preferred Stock are issuable in one or more series. We currently have one series of Class A Preferred Stock, the Series A 3% Preferred Stock (the “3% Preferred Stock”) and one series of Class B Preferred Stock, the Series B-1 8.48% Preferred Stock (the “8.48% Preferred Stock”). At November 20, 2024, 1,435 shares of 3% Preferred Stock and 171,486 shares of 8.48% Preferred Stock were outstanding.

Our Board may, in the future, designate additional series of preferred stock, and fix the relative rights, preferences and limitations of each series. The authorized but unissued shares of our 3% Preferred Stock and 8.48% Preferred Stock and any new series of preferred stock designated by the Board may be issued by the Board in the future.

3% Preferred Stock

Holders of 3% Preferred Stock are entitled to receive an annual dividend of $3.00 per share, which is cumulative and payable quarterly. Holders of 3% Preferred Stock have no preemptive right in, or right to purchase or subscribe for, any additional shares of our capital stock. Dividends or dissolution or liquidation payments to the holders of 3% Preferred Stock must be declared and paid, or set apart for payment, before any dividends or dissolution or liquidation payments can be declared and paid, or set apart for payment, to the holders of 8.48% Preferred Stock or common stock. The 3% Preferred Stock is not convertible into any other of our securities.

8.48% Preferred Stock

Holders of 8.48% Preferred Stock are entitled to receive an annual dividend of $8.48 per share, which is cumulative and payable quarterly. Holders of 8.48% Preferred Stock have no preemptive right in, or right to purchase or subscribe for, any additional shares of our capital stock and have no voting rights. Accumulated dividends on the 8.48% Preferred Stock do not bear interest, and the 8.48% Preferred Stock is not subject to redemption. Dividends or dissolution payments to the holders of 8.48% Preferred Stock must be declared and paid, or set apart for payment, before any dividends or dissolution payments are declared and paid, or set apart for payment, to the holders of common stock. The 8.48% Preferred Stock is not convertible into any other of our securities.

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DESCRIPTION OF DEPOSITARY SHARES

General

We may, at our option, elect to offer fractional shares of preferred stock, which we call depositary shares, rather than full shares of preferred stock. If we do, we will issue to the public receipts, called depositary receipts, for depositary shares, each of which will represent a fraction, to be described in the prospectus supplement, of a share of a particular series of preferred stock.

The shares of any series of preferred stock represented by depositary shares will be deposited with a depositary named in the prospectus supplement. Unless otherwise provided in the prospectus supplement, each owner of a depositary share will be entitled, in proportion to the applicable fractional interest in a share of preferred stock represented by the depositary share, to all the rights and preferences of the preferred stock represented by the depositary share. Those rights include dividend, voting, redemption, conversion and liquidation rights.

Dividends and Other Distributions

The depositary will distribute all cash dividends or other cash distributions received in respect of the preferred stock to the record holders of depositary shares in proportion to the numbers of depositary shares owned by those holders.

If there is a distribution other than in cash, the depositary will distribute property received by it to the record holders of depositary shares, unless the depositary determines that it is not feasible to make the distribution. If this occurs, the depositary may, with our approval, sell the property and distribute the net proceeds from the sale to the holders.

Withdrawal of Stock

Unless the related depositary shares have been previously called for redemption, upon surrender of the depositary receipts at the office of the depositary, the holder of the depositary shares will be entitled to delivery, at the office of the depositary to or upon his or her order, of the number of whole shares of the preferred stock and any money or other property represented by the depositary shares. If the depositary receipts delivered by the holder evidence a number of depositary shares in excess of the number of depositary shares representing the number of whole shares of preferred stock to be withdrawn, the depositary will deliver to the holder at the same time a new depositary receipt evidencing the excess number of depositary shares. In no event will the depositary deliver fractional shares of preferred stock upon surrender of depositary receipts.

Redemption of Depositary Shares

Whenever we redeem shares of preferred stock held by the depositary, the depositary will redeem as of the same redemption date the number of depositary shares representing shares of the preferred stock so redeemed, so long as we have paid in full to the depositary the redemption price of the preferred stock to be redeemed plus an amount equal to any accumulated and unpaid dividends on the preferred stock to the date fixed for redemption. The redemption price per depositary share will be equal to the redemption price and any other amounts per share payable on the preferred stock multiplied by the fraction of a share of preferred stock represented by one depositary share. If less than all the depositary shares are to be redeemed, the depositary shares to be redeemed will be selected by lot or pro rata or by any other equitable method as may be determined by the depositary.

After the date fixed for redemption, depositary shares called for redemption will no longer be deemed to be outstanding and all rights of the holders of depositary shares will cease, except the right to receive the moneys payable upon redemption and any money or other property to which the holders of the depositary shares were entitled upon redemption upon surrender to the depositary of the depositary receipts evidencing the depositary shares.

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Voting the Depositary Shares

Upon receipt of notice of any meeting at which the holders of the preferred stock are entitled to vote, the depositary will mail the information contained in the notice of meeting to the record holders of the depositary receipts relating to that preferred stock. The record date for the depositary receipts relating to the preferred stock will be the same date as the record date for the preferred stock. Each record holder of the depositary shares on the record date will be entitled to instruct the depositary as to the exercise of the voting rights pertaining to the number of shares of preferred stock represented by that holder’s depositary shares. The depositary will endeavor, insofar as practicable, to vote the number of shares of preferred stock represented by the depositary shares in accordance with those instructions, and we will agree to take all action which may be deemed necessary by the depositary in order to enable the depositary to do so. The depositary will not vote any shares of preferred stock except to the extent it receives specific instructions from the holders of depositary shares representing that number of shares of preferred stock.

Charges of Depositary

We will pay all transfer and other taxes and governmental charges arising solely from the existence of the depositary arrangements. We will pay charges of the depositary in connection with the initial deposit of the preferred stock and any redemption of the preferred stock. Holders of depositary receipts will pay other transfer and other taxes and governmental charges and such other charges as are expressly provided in the deposit agreement to be for their accounts.

Resignation and Removal of Depositary

The depositary may resign at any time by delivering to us notice of its election to do so, and we may remove the depositary at any time. Any resignation or removal of the depositary will take effect upon our appointment of a successor depositary and its acceptance of such appointment. The successor depositary must be appointed within 60 days after delivery of the notice of resignation or removal and must be a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50,000,000.

Notices

The depositary will forward to holders of depositary receipts all notices, reports and other communications, including proxy solicitation materials received from us, which are delivered to the depositary and which we are required to furnish to the holders of the preferred stock.

Limitation of Liability

Neither we nor the depositary will be liable if either of us is prevented or delayed by law or any circumstance beyond our control in performing our obligations. Our obligations and those of the depositary will be limited to performance in good faith of our and their duties thereunder. We and the depositary will not be obligated to prosecute or defend any legal proceeding in respect of any depositary shares or preferred stock unless satisfactory indemnity is furnished. We and the depositary may rely upon written advice of counsel or accountants, on information provided by persons presenting preferred stock for deposit, holders of depositary receipts or other persons believed to be competent and on documents believed to be genuine.

DESCRIPTION OF WARRANTS

We may issue warrants to purchase debt securities, preferred stock, depositary shares or common stock. We may offer warrants separately or together with one or more additional warrants, debt securities, preferred stock, depositary shares or common stock, or any combination of those securities in the form of units, as described in the appropriate prospectus supplement. If we issue warrants as part of a unit, the accompanying prospectus

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supplement will specify whether those warrants may be separated from the other securities in the unit prior to the warrants’ expiration date. Below is a description of certain general terms and provisions of the warrants that we may offer. Further terms of the warrants will be described in the prospectus supplement.

The applicable prospectus supplement will contain, where applicable, the following terms of and other information relating to the warrants:

• the specific designation and aggregate number of, and the price at which we will issue, the warrants;

• the currency or currency units in which the offering price, if any, and the exercise price are payable;

• the date on which the right to exercise the warrants will begin and the date on which that right will expire or, if you may not continuously exercise the warrants throughout that period, the specific date or dates on which you may exercise the warrants;

• any applicable anti-dilution provisions;

• any applicable redemption or call provisions;

• the circumstances under which the warrant exercise price may be adjusted;

• whether the warrants will be issued in fully registered form or bearer form, in definitive or global form or in any combination of these forms, although, in any case, the form of a warrant included in a unit will correspond to the form of the unit and of any security included in that unit;

• any applicable material United States federal income tax consequences;

• the identity of the warrant agent for the warrants and of any other depositaries, execution or paying agents, transfer agents, registrars or other agents;

• the proposed listing, if any, of the warrants or any securities purchasable upon exercise of the warrants on any securities exchange;

• the designation and terms of the debt securities, preferred stock, depositary shares or common stock purchasable upon exercise of the warrants;

• the designation, aggregate principal amount, currency and terms of the debt securities that may be purchased upon exercise of the warrants;

• if applicable, the designation and terms of the debt securities, preferred stock, depositary shares or common stock with which the warrants are issued and the number of warrants issued with each security;

• if applicable, the date from and after which the warrants and the related debt securities, preferred stock, depositary shares or common stock will be separately transferable;

• the principal amount of debt securities, the number of shares of preferred stock, the number of depositary shares or the number of shares of common stock purchasable upon exercise of a warrant and the price at which those shares may be purchased;

• if applicable, the minimum or maximum amount of the warrants that may be exercised at any one time;

• information with respect to book-entry procedures, if any;

• whether the warrants are to be sold separately or with other securities as parts of units; and

• any additional terms of the warrants, including terms, procedures and limitations relating to the exchange and exercise of the warrants.

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DESCRIPTION OF PURCHASE CONTRACTS

We may issue purchase contracts, including purchase contracts issued as part of a unit with one or more other securities, for the purchase or sale of our debt securities, preferred stock, depositary shares or common stock. The price of our debt securities or price per share of common stock, preferred stock or depositary shares, as applicable, may be fixed at the time the purchase contracts are issued or may be determined by reference to a specific formula contained in the purchase contracts. We may issue purchase contracts in such amounts and in as many distinct series as we wish.

The applicable prospectus supplement may contain, where applicable, the following information about the purchase contracts issued under it:

• whether the purchase contracts obligate the holder to purchase or sell, or both, our debt securities, common stock, preferred stock or depositary shares, as applicable, and the nature and amount of each of those securities, or method of determining those amounts;

• whether the purchase contracts are to be prepaid or not;

• whether the purchase contracts are to be settled by delivery, or by reference or linkage to the value, performance or level of our common stock or preferred stock;

• any acceleration, cancellation, termination or other provisions relating to the settlement of the purchase contracts;

• United States federal income tax considerations relevant to the purchase contracts; and

• whether the purchase contracts will be issued in fully registered global form.

The applicable prospectus supplement will describe the terms of any purchase contracts. The preceding description and any description of purchase contracts in the applicable prospectus supplement does not purport to be complete and is subject to and is qualified in its entirety by reference to the purchase contract agreement and, if applicable, collateral arrangements and depositary arrangements relating to such purchase contracts.

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DESCRIPTION OF UNITS

We may issue units comprised of two or more of the other securities described in this prospectus in any combination. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date.

The applicable prospectus supplement may describe:

• the designation and terms of the units and of the securities comprising the units, including whether and under what circumstances those securities may be held or transferred separately;

• any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units;

• the terms of the unit agreement governing the units;

• United States federal income tax considerations relevant to the units; and

• whether the units will be issued in fully registered or global form.

The preceding description and any description of units in the applicable prospectus supplement does not purport to be complete and is subject to and is qualified in its entirety by reference to the form of unit agreement which will be filed with the SEC in connection with the offering of such units, and, if applicable, collateral arrangements and depositary arrangements relating to such units.

PLAN OF DISTRIBUTION

We may sell our securities in any of the following ways:

• to or through underwriters;

• through agents;

• through broker-dealers (acting as agent or principal);

• in “at the market” offerings, within the meaning of Rule 415(a)(4) under the Securities Act, to or through a market maker or into an existing trading market on an exchange or otherwise;

• directly by us to purchasers, through a specific bidding or auction process or otherwise; or

• through a combination of any such methods of sale.

Each time that we use this prospectus to sell our securities, we will also provide a prospectus supplement that contains the specific terms of such offering. The prospectus supplement will set forth the terms of the offering of such securities, including:

• the name or names of any underwriters, dealers or agents and the type and amounts of securities underwritten or purchased by each of them;

• the public offering price of the securities and the proceeds to us and any discounts, commissions or concessions allowed or reallowed or paid to underwriters, dealers or agents;

• any exchange on which the securities will be issued; and

• all other items constituting underwriting compensation.

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If we use underwriters in the sale of any securities on a firm commitment basis, the securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The securities may be either offered to the public through underwriting syndicates represented by managing underwriters, or directly by underwriters. Generally, the underwriters’ obligations to purchase the securities will be subject to certain conditions precedent. The underwriters will be obligated to purchase all of the securities if they purchase any of the securities. We may also engage underwriters on a best efforts basis.

We may sell the securities through agents from time to time. The prospectus supplement will name any agent involved in the offer or sale of our securities and any commissions we pay to them. Generally, any agent will be acting on a best efforts basis for the period of its appointment.

To the extent that we make sales to or through one or more underwriters or agents in at-the-market offerings, we will do so pursuant to the terms of a distribution agreement between us and the underwriters or agents. If we engage in at-the-market sales pursuant to a distribution agreement, we will issue and sell shares of our common stock to or through one or more underwriters or agents, which may act on an agency basis or on a principal basis. During the term of any such agreement, we may sell shares on a daily basis in exchange transactions or otherwise as we agree with the underwriters or agents. The distribution agreement will provide that any shares of our common stock sold will be sold at prices related to the then prevailing market prices for our common stock. Therefore, exact figures regarding proceeds that will be raised or commissions to be paid cannot be determined at this time and will be described in a prospectus supplement. Pursuant to the terms of the distribution agreement, we also may agree to sell, and the relevant underwriters or agents may agree to solicit offers to purchase, blocks of our common stock or other securities. The terms of each such distribution agreement will be set forth in more detail in a prospectus supplement to this prospectus. If any underwriter or agent acts as principal, or broker dealer acts as underwriter, it may engage in certain transactions that stabilize, maintain or otherwise affect the price of our securities. We will describe any such activities in the prospectus supplement relating to the transaction.

In the sale of the securities, underwriters or agents may receive compensation from us in the form of underwriting discounts or commissions and may also receive compensation from purchasers of the securities, for whom they may act as agents, in the form of discounts, concessions or commissions. Underwriters may sell the securities to or through dealers, and such dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters and/or commissions from the purchasers for whom they may act as agents. Discounts, concessions and commissions may be changed from time to time. Dealers and agents that participate in the distribution of the securities may be deemed to be underwriters under the Securities Act, and any discounts, concessions or commissions they receive from us and any profit on the resale of securities they realize may be deemed to be underwriting compensation under applicable federal and state securities laws.

We may authorize underwriters, dealers or agents to solicit offers by certain purchasers to purchase our securities at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. The contracts will be subject only to those conditions set forth in the prospectus supplement, and the prospectus supplement will set forth any commissions or discounts we pay for solicitation of these contracts.

Agents and underwriters may be entitled to indemnification by us against certain civil liabilities, including liabilities under the Securities Act of 1933, as amended, or to contribution with respect to payments which the agents or underwriters may be required to make in respect thereof. Agents and underwriters may be customers of, engage in transactions with, or perform services for us in the ordinary course of business.

We may enter into derivative transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If the applicable prospectus supplement indicates in connection with those derivatives then the third parties may sell securities covered by this prospectus and the applicable prospectus supplement, including in short sale transactions. If so, the third party may use securities pledged by us

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or borrowed from us or others to settle those sales or to close out any related open borrowings of stock, and may use securities received from us in settlement of those derivatives to close out any related open borrowings of securities. The third party in such sale transactions will be an underwriter and will be identified in the applicable prospectus supplement (or a post-effective amendment).

Until the distribution of the securities is completed, rules of the SEC may limit the ability of any underwriters and selling group members to bid for and purchase the securities. As an exception to these rules, underwriters are permitted to engage in some transactions that stabilize the price of the securities. Such transactions consist of bids or purchases for the purpose of pegging, fixing or maintaining the price of the securities.

Underwriters may engage in overallotment. If an underwriter creates a short position in offered securities by selling more securities than are set forth on the cover page of the applicable prospectus supplement, the underwriters may reduce that short position by purchasing the securities in the open market.

The lead underwriters may also impose a penalty bid on other underwriters and selling group members participating in an offering. This means that if the lead underwriters purchase securities in the open market to reduce the underwriters’ short position or to stabilize the price of the securities, they may reclaim the amount of any selling concession from the underwriters and selling group members who sold those securities as part of the offering.

LEGAL MATTERS

The validity of the securities offered hereby will be passed upon for us by Luse Gorman, PC, Washington, D.C. Any underwriters will also be advised about the validity of the securities and other legal matters by their own counsel, which will be named in the applicable prospectus supplement.

EXPERTS

The consolidated financial statements of Financial Institutions, Inc. as of December 31, 2023 and 2022 and for each of the years in the three-year period ended December 31, 2023 and the effectiveness of internal control over financial reporting as of December 31, 2023 incorporated in this prospectus by reference from the Financial Institutions, Inc. Annual Report on Form 10-K for the year ended December 31, 2023 have been audited by RSM US LLP, an independent registered public accounting firm, as stated in their reports thereon, incorporated herein by reference, and have been incorporated in this prospectus and registration statement in reliance upon such reports and upon the authority of such firm as experts in accounting and auditing.

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4,000,000 Shares

Common Stock

Prospectus Supplement

Sole-Bookrunner

Keefe, Bruyette & Woods

A Stifel Company

Co-Manager

Piper Sandler

December 11, 2024