Arthur J. Gallagher & Co. (AJG) 424B2 Prospectus for primary offering

Filed Pursuant to Rule 424(b)(2)
Registration No. 333-277002

PROSPECTUS SUPPLEMENT

(To prospectus dated February 12, 2024)

3,000,000 shares

Arthur J. Gallagher & Co.

Common Stock

We have entered into an equity distribution agreement (the “equity distribution agreement”) with Morgan Stanley & Co. LLC (the “agent”) relating to the issuance and sale, from time to time, of up to 3,000,000 shares of our common stock, par value $1.00 per share (our “common stock”), offered by this prospectus supplement and the accompanying prospectus. These sales, if any, will be made pursuant to the terms of the equity distribution agreement dated March 14, 2024 between us and the agent.

Sales of shares of our common stock, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions, which may include block trades, or transactions that are deemed to be “at the market” offerings as defined in Rule 415(a)(4) under the Securities Act of 1933, as amended (the “Securities Act”), including sales made directly on the New York Stock Exchange (the “NYSE”), or sales made to or through a market maker other than on an exchange or as otherwise agreed upon by the agent and us. Subject to the terms and conditions of the equity distribution agreement, the agent will use its commercially reasonable efforts consistent with its normal trading and sales practices to sell the shares of common stock on our behalf, on mutually agreed terms between such agent and us. We also may sell some or all of the shares in this offering to an agent as principal for its own account at a price per share agreed upon at the time of sale. If we sell shares of our common stock to an agent as principal, we will enter into a separate agreement with such agent setting forth the terms of such transaction, and, to the extent required by applicable law, we will describe this agreement in a separate prospectus supplement or pricing supplement.

We will pay the agent a commission at a mutually agreed-upon rate that will not exceed, but may be lower than, 1.25% of the gross sales price of all shares sold through it as agent under the equity distribution agreement. In connection with the sale of the shares of common stock on our behalf, the agent may be deemed to be an “underwriter” within the meaning of the Securities Act, and the agent’s compensation may be deemed to be underwriting commissions or discounts. See “Plan of Distribution” beginning on page S-10 for additional information regarding the compensation to be paid to the agent.

Our common stock is listed on the NYSE under the symbol “AJG.” On March 13, 2024, the last reported sale price of our common stock on the NYSE was $255.38 per share. You are urged to obtain current market data and should not use the market price as of March 13, 2024 as a prediction of the future market price of our common stock. The net proceeds from any sales under this prospectus supplement will be used as described under “Use of Proceeds.”

Investing in our common stock involves risks. See “Risk Factors” beginning on page S-3, of this prospectus supplement and page 3 of the accompanying prospectus. You should also consider the risk factors described in the documents incorporated by reference into this prospectus supplement and the accompanying prospectus, including the risk factors described in our Annual Report on Form 10-K.

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

Morgan Stanley

The date of this prospectus supplement is March 14, 2024.

TABLE OF CONTENTS

Prospectus Supplement

	Page
ABOUT THIS PROSPECTUS SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS		S-ii
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS		S-iii
PROSPECTUS SUPPLEMENT SUMMARY		S-1
THE OFFERING		S-2
RISK FACTORS		S-3
USE OF PROCEEDS		S-5
CERTAIN U.S. FEDERAL INCOME TAX CONSIDERATIONS FOR NON-U.S. HOLDERS		S-6
PLAN OF DISTRIBUTION		S-10
VALIDITY OF THE COMMON STOCK		S-12
EXPERTS		S-12
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE		S-13

Prospectus

	Page
ABOUT THIS PROSPECTUS		1
ARTHUR J. GALLAGHER & CO.		2
RISK FACTORS		3
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS		4
USE OF PROCEEDS		8
SELLING SECURITYHOLDERS		9
DESCRIPTION OF SECURITIES		10
PLAN OF DISTRIBUTION		11
LEGAL MATTERS		12
EXPERTS		13
WHERE YOU CAN FIND MORE INFORMATION		14
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE		15

You should rely only on the information contained in or incorporated by reference in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference therein or that is contained in any free writing prospectus prepared by us or on our behalf. We have not, and the underwriters have not, authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We and the agent take no responsibility for, and can provide no assurances as to the reliability of, any other information that others may give you. We are not, and the agent is not, making an offer to sell the common stock in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus supplement and the accompanying prospectus and in the documents incorporated by reference herein and therein or that is contained in any free writing prospectus issued by us is accurate only as of their respective dates. Our business, financial condition, results of operation and prospects may have changed since those dates.

ABOUT THIS PROSPECTUS SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS

We provide information to you about the common stock in two separate documents: (1) this prospectus supplement, which describes the specific terms of the common stock and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference in that prospectus and (2) the accompanying prospectus, which provides general information about securities we may offer from time to time, including securities other than the common stock being offered by this prospectus supplement. If information in this prospectus supplement is inconsistent with the accompanying prospectus, you should rely on this prospectus supplement.

It is important for you to read and consider all of the information contained in this prospectus supplement and the accompanying prospectus in making your investment decision. You also should read and consider the information in the documents we have referred you to in “Where You Can Find Additional Information” in this prospectus supplement and the accompanying prospectus.

We include cross-references in this prospectus supplement and the accompanying prospectus to captions in these materials where you can find additional related discussions. The table of contents in this prospectus supplement provides the pages on which these captions are located.

Unless otherwise indicated or the context otherwise requires, references in this prospectus supplement to the “Company,” “we,” “our,” and “us” refer to both Arthur J. Gallagher & Co. and its consolidated subsidiaries, and “AJG” refers to Arthur J. Gallagher & Co., a Delaware corporation. The term “you” refers to a prospective investor.

S-ii

CAUTIONARY STATEMENT REGARDING

FORWARD-LOOKING STATEMENTS

Statements contained or incorporated by reference in this prospectus supplement and the accompanying prospectus and the documents they incorporate by reference, contain certain statements related to future results, or state our intentions, beliefs and expectations or predictions for the future, which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. Forward-looking statements relate to expectations or forecasts of future events. Such statements use words such as “anticipate,” “believe,” “estimate,” “expect,” “contemplate,” “forecast,” “project,” “intend,” “plan,” “potential,” and other similar terms, and future or conditional tense verbs like “could,” “may,” “might,” “see,” “should,” “will” and “would.” You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as: the impact of general economic conditions, including significant inflation, interest rates and market uncertainty; the effects of geopolitical volatility, including repercussions from the wars in Ukraine and the Middle East; market and industry conditions, including competitive and pricing trends; acquisition strategy including the expected size of our acquisition program; the expected impact of acquisitions and dispositions and integrating recent acquisitions, including comments regarding the expected benefits of our acquisition of the Willis Towers Watson plc treaty reinsurance brokerage operations (which we refer to as Willis Re), BCHR Holdings, L.P., and its subsidiaries, dba Buck (which we refer to as Buck), Cadence Insurance, Inc. (which we refer to as Cadence Insurance), Eastern Insurance Group, LLC (which we refer to as Eastern Insurance), My Plan Manager Group Pty Ltd (which we refer to as My Plan Manager), and other acquisitions larger than our typical tuck-in acquisitions and the expected duration and costs of integrating such large acquisitions; the development and performance of our services and products; changes in the composition or level of our revenues or earnings; our cost structure and the size and outcome of cost-saving or restructuring initiatives; future capital expenditures; future debt levels and anticipated actions to be taken in connection with maturing debt; future debt to earnings ratios; the outcome of contingencies; dividend policy; pension obligations; cash flow and liquidity; capital structure and financial losses; future actions by regulators; the outcome of existing regulatory actions, audits, reviews or litigation; the impact of changes in accounting rules; financial markets; interest rates; foreign exchange rates; matters relating to our operations; income taxes; expectations regarding our investments; human capital management, including diversity and inclusion initiatives, and environmental, social and governance matters, including climate-resilience and climate-advising products and services and carbon emissions. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors.

Potential factors that could impact results include:

• Global economic and geopolitical events, such as high inflation and related monetary policy responses including increased interest rates; a recession or economic downturn; failures of financial institutions and other counterparties or a potential United States (U.S.) government shutdown or gridlock over increasing the U.S. debt ceiling; political violence, and instability, including geo-economic fragmentation;

• Economic conditions that result in financial difficulties for underwriting enterprises or lead to reduced risk-taking capital capacity, for example, as a result of large payouts related to extreme weather events, or to the failure of such enterprises, including the increased risk of errors and omissions (which we refer to as E&O) claims against us;

• Risks that could negatively affect the success of our acquisition strategy, including the impact of current economic uncertainty on our ability to source, review and price acquisitions, continuing consolidation in our industry and interest in acquiring insurance brokers on the part of private equity firms and newly public insurance brokers, which makes it more difficult to identify targets and in some cases makes them more expensive, inaccurate assumptions and failure to realize expected benefits; the risk that we may not receive timely regulatory approval of pending transactions, closing risks; execution risks, integration risks, poor cultural fit, the risk of post-acquisition deterioration leading to

S-iii

intangible asset impairment charges, and the risk we could incur or assume unanticipated liabilities such as cybersecurity issues or those relating to violations of anti-corruption and sanctions laws;

• Risks related to Willis Re, Buck, Cadence Insurance, Eastern Insurance, My Plan Manager and other acquisitions larger than our usual tuck-in acquisitions, including risks related to our ability to successfully integrate operations, the possibility that our assumptions may be inaccurate resulting in unforeseen obligations or liabilities and failure to realize the expected benefits of these acquisitions;

• Damage to our reputation, including as a result of environmental, social and governance (which we refer to as ESG) matters and the potential for the Internet and social media to magnify the effects of such reputational issues;

• Failure to meet our sustainability and ESG-related aspirations, goals and initiatives or to comply with increasingly complex climate-related regulations, including increased risks related to “greenwashing”;

• Emerging risks relating to the use of artificial intelligence (which we refer to as AI) in our business operations, including regulatory, data privacy and cybersecurity risks;

• Failure to apply technology, data analytics and AI effectively in driving value for our clients through technology-based solutions, or failure to gain internal efficiencies and effective internal controls through the application of technology and related tools;

• Risks associated with the use of AI in our business operations, including regulatory, data privacy, cybersecurity, E&O and competition risks;

• Failure to attract and retain experienced and qualified talent, including our senior management team, or adequately plan and execute for the succession of such leaders; increased costs resulting from increased compensation and benefits packages as a result of a tighter labor market, and negative effects from restrictions on non-competes at the state and federal level;

• Substantial increase in remote work among our employees, which may affect our corporate culture, productivity, collaboration and effective communication, increase cybersecurity or data breaches risks, heighten vulnerability to solicitations by competing firms and impact our ability to recruit and retain employees that prefer fully remote or fully-in-person work environments;

• A disaster or other significant disruption to business continuity for our own operations or those of third-parties on which we rely, including cybersecurity incidents; natural disasters; political violence and unrest in the U.S. or elsewhere around the world; for example, our substantial operations in India could be negatively impacted as a result of the dispute between India and Pakistan involving the Kashmir region, rising tensions between India and China, or incidents of terrorism in India, civil unrest or other reasons;

• Sustained increases in the cost of employee benefits and compensation expense;

• Risks arising from our international operations and changes in international conditions, including the risks posed by political and economic uncertainty in certain countries (including repercussions from the wars in Ukraine and the Middle East), risks related to maintaining regulatory and legal compliance across multiple jurisdictions (such as those relating to violations of anti-corruption, sanctions, protectionism, privacy laws and increasingly complex regulatory requirements related to climate change and sustainability issues), as well as, risks related to tariffs, trade wars, or climate change and other long-term environmental, social and governance matters and global health risks;

• Risks related to changes in U.S. or foreign tax laws, including a U.S. or foreign tax rate change, potential changes in guidance related to the U.S. Inflation Reduction Act, the Organisation for Economic Co-operation and Development’s (OECD) global minimum corporate tax regime, and other local policy changes;

S-iv

• Competitive pressures, including as a result of innovation, in each of our businesses;

• Volatility or declines in premiums or other adverse trends in the insurance industry;

• The higher level of variability inherent in contingent and supplemental revenues versus standard commission revenues;

• Risks particular to our benefit consulting operations, including risks related to the acquisition of Buck;

• Risks particular to our third-party claims administrations operations, including risks related to the availability of RISX-FACS^®, our proprietary risk management information system, wage inflation, staffing shortages, any slowing of the trend toward outsourcing claims administration, and the concentration of large amounts of revenue with certain clients;

• Climate risks, including the risk of a systemic economic crisis and disruptions to our business caused by the transition to a low-carbon economy;

• Cyber-attacks or other cybersecurity incidents such as the ransomware incident we publicly disclosed in September 2020 and the heightened risk of such attacks as a result of the wars in Ukraine and the Middle East, improper disclosure of confidential, personal or proprietary data and changes to laws and regulations governing cybersecurity and data privacy;

• Unfavorable determinations related to contingencies and legal proceedings;

• Violations or alleged violations of the U.S. Foreign Corrupt Practices Act, the United Kingdom (U.K.) Bribery Act 2010 or other anti-corruption laws and the Foreign Account Tax Compliance provisions of the Hiring Incentives to Restore Employment Act, and the outcome of any existing or future investigation, review, regulatory action or litigation;

• Failure to comply with regulatory requirements, including those related to governance and control requirements in particular jurisdictions, international sanctions, including new sanctions laws as a result of the wars in Ukraine and the Middle East; laws relating to the disclosure of ESG-related matters; laws relating to the use of AI, or a change in regulations or enforcement policies that adversely affects our operations (for example, relating to insurance broker compensation methods or restrictions on non-competes);

• Changes to our financial presentation from new accounting estimates and assumptions;

• Intellectual property risks;

• Risks related to our legacy clean energy investments, including intellectual property claims, environmental and product liability claims, environmental compliance costs and the risk of disallowance by the Internal Revenue Service of previously claimed tax credits;

• The risk that our outstanding debt adversely affects our financial flexibility and restrictions and limitations in the agreements and instruments governing our debt;

• The risk of credit rating downgrades;

• The risk we may not be able to receive dividends or other distributions from subsidiaries, including the effects of significant changes in foreign exchange rates;

• The risk of share ownership dilution when we issue common stock; and

• Volatility of the price of our common stock.

Forward-looking statements are not guarantees of future performance. They involve risks, uncertainties and assumptions, including the risk factors referred to above. Our future performance and actual results or outcomes may differ materially from those expressed in forward-looking statements. Accordingly, you should not place

S-v

undue reliance on forward-looking statements, which speak only as of, and are based on information available to us on, the date of the applicable document. Many of the factors that will determine these results are beyond our ability to control or predict. All subsequent written and oral forward-looking statements attributable to us or any person acting on our behalf are expressly qualified in their entirety by the cautionary statements contained or referred to in this section. Forward-looking statements speak only as of the date that they are made, and we do not undertake any obligation to update any such statements or release publicly any revisions to these forward-looking statements to reflect events or circumstances after the date of this prospectus supplement or to reflect new information, future or unexpected events or otherwise, except as required by applicable law or regulation. In addition, historical, current and forward-looking sustainability-related or ESG-related statements may be based on standards for measuring progress that are still developing, internal controls and processes that continue to evolve, and assumptions that are subject to change in the future.

For additional factors you should consider, please see “Risk Factors” below and in the accompanying prospectus and in our filings with the SEC, including our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, and any reports we file with the SEC in the future, which are incorporated by reference into this prospectus supplement.

S-vi

PROSPECTUS SUPPLEMENT SUMMARY

The following summary should be read together with the information contained in other parts of this prospectus supplement and the accompanying prospectus and the information incorporated by reference herein or therein. This summary highlights selected information from this prospectus supplement and the accompanying prospectus regarding the offering of the shares of common stock. You should read this prospectus supplement and the accompanying prospectus in their entirety, including the documents we incorporate by reference, carefully to understand fully the terms of the offering as well as other considerations that are important to you in making a decision to invest in the shares of our common stock. You should pay special attention to the “Risk Factors” section of this prospectus supplement, and the “Risk Factors” section in our annual report on Form 10-K for the fiscal year ended December 31, 2023 to determine whether an investment in the shares of common stock is appropriate for you.

OUR COMPANY

We are engaged in providing insurance brokerage, reinsurance brokerage, consulting, and third-party property/casualty claims settlement and administration services to entities and individuals around the world. We believe that our major strength is our ability to deliver comprehensively structured insurance, reinsurance and risk management solutions, superior claim outcomes and comprehensive consulting services to our clients. We report our results in three segments: brokerage, risk management and corporate. The major sources of our operating revenues are commissions, fees and supplemental and contingent revenues from our brokerage operation, and fees, including performance-based fees, from our risk management operations. The corporate segment generated revenues from our clean energy investments through 2022.

CORPORATE INFORMATION

Our principal executive offices are located at 2850 Golf Road, Rolling Meadows, Illinois 60008-4050. Our telephone number is (630) 773-3800. Our website is http://www.ajg.com. The contents of our website are not a part of this prospectus supplement or the accompanying prospectus.

S-1

THE OFFERING

Issuer Arthur J. Gallagher & Co.

Common stock offered by us Up to 3,000,000 shares of common stock

Manner of Offering Sales of shares of our common stock, if any, under this prospectus supplement and the accompanying prospectus may be made in negotiated transactions, which may include block trades, or transactions that are deemed to be “at the market” offerings as defined in Rule 415(a)(4) under the Securities Act, including sales made directly on the NYSE, or sales made to or through a market maker other than on an exchange or as otherwise agreed upon by the agent and us. Subject to the terms and conditions of the equity distribution agreement, the agent will use its commercially reasonable efforts consistent with its normal trading and sales practices to sell the shares of common stock on our behalf, on mutually agreed terms between such agent and us. We also may sell some or all of the shares in this offering to an agent as principal for its own account at a price per share agreed upon at the time of sale and subject to a separate agreement.

We or the agent may suspend the offering of shares of our common stock upon proper notice and subject to other conditions.

Use of proceeds We intend to use the net proceeds of this offering, after deducting the agent’s commissions and our offering expenses, for general corporate purposes, which may include acquisitions, repayment of outstanding indebtedness, capital expenditures, working capital, and stock repurchases.

Listing Our common stock is listed on the NYSE under the symbol “AJG.”

Transfer Agent and Registrar Computershare Trust Company, N.A.

U.S. Federal Income Tax Considerations The federal income tax consequences of purchasing, owning and disposing of our common stock are described in “Material U.S. Federal Income Tax Consequences.”

Risk Factors Investing in our common stock involves risks. See “Risk Factors” on page S-3 of this prospectus supplement and other information included and incorporated by reference in this prospectus supplement and the accompanying prospectus for a discussion of the factors you should consider carefully before deciding to invest in our common stock.

S-2

RISK FACTORS

Investing in our common stock involves risks. Our business is influenced by many factors that are difficult to predict and beyond our control and that involve uncertainties that may materially affect our results of operations, financial condition or cash flows, or the value of our common stock. These risks and uncertainties include those described below, as well as in the risk factors and other sections of the documents that are incorporated by reference in this prospectus supplement and the accompanying prospectus, including the risk factors in our Annual Report on Form 10-K for the fiscal year ended December 31, 2023. You should carefully consider these risks and uncertainties and all of the information contained or incorporated by reference in this prospectus supplement and the accompanying prospectus before you invest in our common stock.

The price of our common stock may fluctuate significantly, and this may make it difficult for you to resell shares of common stock owned by you at times or at prices you find attractive.

The trading price of our common stock may fluctuate widely as a result of a number of factors, many of which are outside our control. In addition, the stock market is subject to fluctuations in the share prices and trading volumes that affect the market prices of the shares of many companies. These broad market fluctuations have adversely affected and may continue to adversely affect the market price of our common stock. Among the factors that could affect our stock price are:

• general economic and political conditions such as recessions, economic downturns or other factors like new pandemics, inflation, worsening international relations, the wars in Ukraine and the Middle East, tariffs, trade wars, political violence and unrest in the U.S. or around the world, or climate change and other long-term environmental, social and governance matters and global health risks;

• fluctuations in our financial results, revenues and expenses;

• quarterly variations in our operating results;

• seasonality of our business cycle;

• changes in the market’s expectations about our operating results;

• our ability to control expenses;

• loss of revenues;

• foreign currency and exchange rate fluctuations;

• our operating results failing to meet the expectation of securities analysts or investors in a particular period;

• changes in financial estimates and recommendations by securities analysts concerning us or the financial or professional services industries in general;

• operating and stock price performance of other companies that investors deem comparable to us;

• news reports relating to trends in our markets, including any expectations regarding an upcoming “hard” or “soft” market;

• changes in laws and regulations affecting our business;

• material announcements by us or our competitors;

• the impact or perceived impact of developments relating to our acquisitions, including the possible perception by securities analysts or investors that such acquisitions divert management attention from our core operations;

• market volatility;

S-3

• general conditions in the insurance industry, including changes or consolidation in the insurance brokerage industry, the volatility of or decline in premiums or other adverse trends in the insurance industry;

• our investments in geographic expansion and to increase our presence in existing markets;

• our ability to successfully execute our strategic growth plans;

• cyber-attacks or other cybersecurity incidents;

• legal proceedings;

• regulatory requirements, including international sanctions and the U.S. Foreign Corrupt Practices Act, the UK Bribery Act 2010 or other anti-corruption laws and the Foreign Account Tax Compliance provisions of the Hiring Incentives to Restore Employment Act;

• changes in accounting policies or practices that may affect our level of net income; and

• sales of substantial amounts of common shares by our directors, executive officers or significant stockholders or the perception that such sales could occur.

In recent years, the stock market has experienced significant price and volume fluctuations. This volatility frequently has occurred without regard to the operating performance of the affected companies. Hence, the price of our common stock could fluctuate based upon factors that have little or nothing to do with us, and these fluctuations could materially reduce our share price.

Sales of substantial amounts of our common stock in the public market, or the perception that these sales may occur, could cause the market price of our common stock to decline.

Sales of substantial amounts of our common stock in the public market, or the perception that these sales may occur could cause the market price of our common stock to decline. This could also impair our ability to raise additional capital through the sale of our equity securities. We may issue additional common stock in the future in connection with capital raisings, acquisitions, strategic transactions or for other purposes. To the extent we issue substantial additional common stock, the ownership of our existing stockholders would be diluted and our earnings per share could be reduced, which may negatively affect the market prices for our common stock.

The common stock offered hereby may be sold in “at the market offerings,” and investors who buy shares at different times will likely pay different prices.

Investors who purchase shares in this offering at different times will likely pay different prices, and accordingly may experience different levels of dilution and different outcomes in their investment results. We will have discretion, subject to market demand, to vary the timing, prices and number of shares sold in this offering. Investors may experience a decline in the value of the shares they purchase in this offering as a result of sales made at prices lower than the prices they paid.

The common stock is equity and is subordinate to our existing and future indebtedness and preferred stock.

Shares of our common stock are equity interests in the Company and do not constitute indebtedness. As such, shares of our common stock will rank junior to all indebtedness and other non-equity claims on the Company with respect to assets available to satisfy claims on the Company, including in a liquidation of the Company. Additionally, our board of directors is authorized to issue series of preferred stock without any action on the part of holders of our common stock. Holders of our common stock are subject to the prior dividend and liquidation rights of any holders of our preferred stock or depositary shares representing such preferred stock then outstanding.

S-4

USE OF PROCEEDS

We intend to use the net proceeds of this offering, after deducting the agent’s commissions and our offering expenses, for general corporate purposes, which may include acquisitions, repayment of outstanding indebtedness, capital expenditures, working capital, and stock repurchases.

S-5

CERTAIN U.S. FEDERAL INCOME TAX CONSIDERATIONS FOR NON-U.S. HOLDERS

The following is a summary of the material U.S. federal income tax consequences applicable to non-U.S. holders (as defined below) with respect to the acquisition, ownership and disposition of shares of our common stock, but does not purport to be a complete analysis of all potential tax considerations related thereto. This summary is based on current provisions of the Internal Revenue Code of 1986, as amended, (the “Code”), final, temporary and proposed Treasury regulations promulgated thereunder, administrative rulings and judicial opinions, all of which are subject to change, possibly with retroactive effect. We have not sought any ruling from the U.S. Internal Revenue Service (the “IRS”), with respect to the statements made and the conclusions reached in the following summary, and there can be no assurance that the IRS will agree with such statements and conclusions.

This summary is limited to non-U.S. holders who purchase shares of our common stock issued pursuant to this offering and who hold such shares of our common stock as capital assets within the meaning of Section 1221 of the Code. This discussion does not address all aspects of U.S. federal income taxation that may be important to a particular non-U.S. holder in light of that non-U.S. holder’s individual circumstances, nor does it address any aspects of U.S. federal estate or gift tax laws or tax considerations arising under the laws of any non-U.S., state or local jurisdiction.

This discussion also does not address tax considerations applicable to a non-U.S. holder subject to special treatment under the U.S. federal income tax laws, including, without limitation:

• banks, insurance companies or other financial institutions;

• partnerships or other pass-through entities;

• tax-exempt organizations;

• tax-qualified retirement plans;

• dealers in securities or currencies;

• traders in securities that elect to use a mark-to-market method of accounting for their securities holdings;

• certain U.S. expatriates;

• controlled foreign corporations;

• passive foreign investment companies;

• persons that own, or have owned, actually or constructively, more than 5% of our common stock; and

• persons that will hold common stock as a position in a hedging transaction, “straddle” or “conversion transaction” for tax purposes.

Accordingly, we urge you to consult with your own tax advisors regarding the U.S. federal, state, local and non-U.S. income and other tax considerations of acquiring, holding and disposing of shares of our common stock.

If a partnership (or other entity or arrangement treated as a partnership for U.S. federal income tax purposes) is a beneficial owner of shares of our common stock, the tax treatment of a partner in such partnership will generally depend upon the status of the partner and the activities of the partnership. Any partner in a partnership holding shares of our common stock (and such partnership) should consult their own tax advisors.

YOU ARE URGED TO CONSULT YOUR TAX ADVISORS WITH RESPECT TO THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO THEIR PARTICULAR SITUATIONS, AS WELL AS ANY

S-6

TAX CONSEQUENCES OF THE ACQUISITION, OWNERSHIP AND DISPOSITION OF SHARES OF OUR COMMON STOCK ARISING UNDER THE U.S. FEDERAL ESTATE OR GIFT TAX RULES OR UNDER THE LAWS OF ANY STATE, LOCAL OR NON-U.S. TAXING JURISDICTION OR UNDER ANY APPLICABLE TAX TREATY.

Definition of Non-U.S. Holder

In general, a “non-U.S. holder” is any beneficial owner of shares of our common stock (other than a partnership or other entity or arrangement treated as a partnership for U.S. federal income tax purposes) that is not a U.S. person. A “U.S. person” is a beneficial owner of shares of our common stock that, for U.S. federal income tax purposes, is:

• an individual citizen or resident of the United States;

• a corporation created or organized in or under the laws of the United States, any state thereof or the District of Columbia;

• an estate, the income of which is subject to U.S. federal income tax regardless of its source; or

• a trust if (a) a court within the United States is able to exercise primary supervision over the administration of the trust and one or more U.S. persons have the authority to control all substantial decisions of the trust or (b) it has in effect a valid election under applicable Treasury regulations to be treated as a U.S. person.

Distributions on Our Common Stock

Distributions, if any, on our common stock will generally constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. If a distribution exceeds our current and accumulated earnings and profits, the excess will be treated first as reducing your adjusted basis in your shares of common stock, and, to the extent it exceeds such adjusted basis, as capital gain from the sale or exchange of such common stock, which will be treated as described under the section titled “—Gain on Sale or Other Taxable Disposition of Shares of Our Common Stock” below.

Dividends paid to a non-U.S. holder on shares of our common stock will generally be subject to U.S. federal withholding tax at a rate of 30%, or such lower rate as may be specified by an applicable income tax treaty. To receive the benefit of a reduced treaty rate, a non-U.S. holder must furnish to us or our paying agent a valid IRS Form W-8BEN (or other applicable form) certifying, under penalties of perjury, such non-U.S. holder’s qualification for the reduced rate.

If a non-U.S. holder holds shares of our common stock in connection with the conduct of a trade or business in the United States, and dividends paid on shares of our common stock are effectively connected with such non-U.S. holder’s U.S. trade or business (and, if required by an applicable income tax treaty, are attributable to a permanent establishment maintained by the non-U.S. holder in the United States), the non-U.S. holder will be exempt from the aforementioned U.S. federal withholding tax. To claim the exemption, the non-U.S. holder must furnish to us or our paying agent a properly executed IRS Form W-8ECI (or applicable successor form). Such effectively connected dividends will generally be subject to U.S. federal income tax on a net income basis at the same graduated U.S. federal income tax rates applicable to U.S. persons. A non-U.S. holder that is a corporation may also be subject to an additional branch profits tax equal to 30% (or such lower rate as may be specified by an applicable income tax treaty) on its effectively connected earnings and profits for the taxable year, as adjusted.

A non-U.S. holder that claims an exemption from withholding or the benefit of an applicable income tax treaty will generally be required to satisfy applicable certification and other requirements prior to the distribution date. Non-U.S. holders that do not timely provide us or our paying agent with the required certification may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS.

S-7

Gain on Sale or Other Taxable Disposition of Shares of Our Common Stock

Subject to the discussion below regarding backup withholding, a non-U.S. holder will not generally be subject to U.S. federal income tax on any gain realized upon the sale or other taxable disposition of shares of our common stock, unless:

• the gain is effectively connected with a trade or business carried on by the non-U.S. holder in the United States and, if required by an applicable income tax treaty, the gain is attributable to a permanent establishment of the non-U.S. holder maintained in the United States;

• the non-U.S. holder is an individual present in the United States for 183 days or more in the taxable year of disposition and certain other requirements are met; or

• our common stock constitutes a “U.S. real property interest” by reason of our status as 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 your disposition of, or your holding period for, our common stock, and, in the case where shares of our common stock are regularly traded on an established securities market, you own, or are treated as owning, more than 5% of our common stock at any time during the foregoing period.

We believe we currently are not, and we do not anticipate becoming, a USRPHC for U.S. federal income tax purposes.

Gain described in the first bullet point above will be subject to U.S. federal income tax on a net income basis at the same graduated U.S. federal income tax rates applicable to U.S. persons. A non-U.S. holder that is a corporation may also be subject to an additional branch profits tax equal to 30% (or such lower rate as may be specified by an applicable income tax treaty) on its effectively connected earnings and profits for the taxable year, as adjusted. Non-U.S. holders should consult any applicable income tax treaties that may provide for different rules.

Gain described in the second bullet point above will be subject to U.S. federal income tax at a flat 30% rate (or such lower rate as may be specified by an applicable income tax treaty), but may be offset by U.S. source capital losses realized during the same taxable year. Non-U.S. holders should consult any applicable income tax treaties that may provide for different rules.

Backup Withholding and Information Reporting

Generally, we must report annually to the IRS and to each non-U.S. holder the amount of dividends paid to, and the tax withheld with respect to, each non-U.S. holder. This information may also be made available to the tax authorities of the country in which the non-U.S. holder resides or is incorporated under the provisions of a specific treaty or agreement. Backup withholding will not generally apply to distributions to a non-U.S. holder provided the non-U.S. holder furnishes to us or our paying agent the required certification as to its non-U.S. status, such as by providing a valid IRS Form W-8BEN, W-8BEN-E or IRS Form W-8ECI, or certain other requirements are met. Notwithstanding the foregoing, backup withholding may apply if either we or our paying agent has actual knowledge, or reason to know, that the holder is a U.S. person that is not an exempt recipient.

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

S-8

Foreign Accounts Tax Compliance Act

Under Sections 1471 through 1474 of the Code (such Sections commonly referred to as “FATCA”), a 30% United States federal withholding tax may apply to any dividends paid to (i) a “foreign financial institution” (as specifically defined in the Code) which does not provide sufficient documentation, typically on IRS Form W-8BEN-E, evidencing either (x) an exemption from FATCA, or (y) its compliance (or deemed compliance) with FATCA (which may alternatively be in the form of compliance with an intergovernmental agreement with the United States) in a manner which avoids withholding, or (ii) a “non-financial foreign entity” (as specifically defined in the Code) which does not provide sufficient documentation, typically on IRS Form W-8BEN-E, evidencing either (x) an exemption from FATCA, or (y) adequate information regarding certain substantial United States beneficial owners of such entity (if any). You are urged to consult your own tax advisor regarding these rules.

S-9

PLAN OF DISTRIBUTION

We have entered into an equity distribution agreement, dated as of March 14, 2024 with Morgan Stanley & Co. LLC, the agent, under which we may from time to time offer and sell up to 3,000,000 shares of our common stock. Sales of shares of our common stock, if any, under this prospectus supplement and the accompanying prospectus may be made in transactions that are deemed to be “at the market” offerings as defined in Rule 415(a)(4) under the Securities Act, including sales made directly on the NYSE or sales made to or through a market maker other than on an exchange or as otherwise agreed upon by the agent and us, or in negotiated transactions, which may include block trades. As sales agent, the agent will not engage in any prohibited stabilizing transactions in connection with the sale of our common stock.

The offering of the shares of our common stock by the agent is subject to receipt and acceptance of written instructions and subject to the agent’s right to decline any order in whole or in part. Upon its acceptance of a placement notice from us, the agent will use its commercially reasonable efforts consistent with its customary trading and sales practices to solicit offers to purchase shares of our common stock up to the amount specified, and in accordance with the terms and conditions set forth in the equity distribution agreement. The placement notice that we deliver will set forth the number of shares to be issued, the time period during which sales are requested to be made, any limitation on the number of shares that may be sold in any one trading day and any minimum price below which sales may not be made. We or the agent may suspend the offering of shares of our common stock upon proper notice and subject to other conditions.

The agent will provide written confirmation to us no later than the opening of the trading day on the NYSE on the day following the trading day on which shares of our common stock were sold under the equity distribution agreement. Each confirmation will include the number of shares sold on the relevant day, the gross proceeds received from such sale and the compensation payable by us to the agent in connection with such sales.

We will pay the agent’s commissions for its services in acting as agent and/or principal in the sale of shares of our common stock. We will pay the agent a commission at a mutually agreed-upon rate that will not exceed, but may be lower than, 1.25% of the gross sales price of all shares sold through it as agent under the equity distribution agreement. The Agent has agreed to reimburse us for certain expenses we incur in connection with the offering. We estimate that the expenses of this offering payable by us, excluding compensation payable to the agent under the terms of the equity distribution agreement, will be approximately $500,000, inclusive of the SEC registration fee.

We may also sell shares of our common stock to the agent as principal for its own account at a price agreed upon at the time of sale. If we sell shares of our common stock to an agent as principal, we will enter into a separate agreement with such agent setting forth the terms of such transaction, and, to the extent required by applicable law, we will describe this agreement in a separate prospectus supplement or pricing supplement.

We will report at least quarterly the number of shares of our common stock sold through the agent under the equity distribution agreement, the net proceeds to us and the compensation paid by us to the agent in connection with the sales of shares of our common stock.

In connection with the sale of shares of our common stock on our behalf, the agent may be deemed to be an “underwriter” within the meaning of the Securities Act and the compensation of the agent may be deemed to be underwriting commissions or discounts. We have agreed to indemnify the agent against certain civil liabilities, including liabilities under the Securities Act, or to contribute with respect to payments that the agent may be required to make because of those liabilities.

The agent and its affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory investment management, investment research, principal investment, hedging, financing and brokerage activities. The agent and its affiliates have provided, and may in the future provide, various investment banking, commercial banking,

S-10

fiduciary and advisory services to us from time to time for which they have received, and may in the future receive, customary fees and reimbursement of expenses. The agent and its affiliates may, from time to time, engage in other transactions with and perform services for us in the ordinary course of their business for which they may receive customary fees and reimbursement of expenses.

In the ordinary course of their business activities, the agent and its 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. Such investments and securities activities may involve securities and/or instruments of ours or our affiliates. The agent and its affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or financial instruments and may hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

The offering of shares of our common stock pursuant to the equity distribution agreement will terminate upon the earlier of (1) the sale of all of the shares subject to the equity distribution agreement and (2) the termination of the equity distribution agreement pursuant to its terms.

S-11

VALIDITY OF THE COMMON STOCK

The validity of the shares of common stock offered hereby will be passed upon for us by Gibson, Dunn & Crutcher LLP. Certain legal matters in connection with this offering will be passed upon for the agent by Cleary Gottlieb Steen & Hamilton LLP, New York, New York.

EXPERTS

The consolidated financial statements and related financial statement schedule of Arthur J. Gallagher & Co. appearing in Arthur J. Gallagher & Co.’s Annual Report (Form 10-K) for the year ended December 31, 2023, and the effectiveness of Arthur J. Gallagher & Co.’s internal control over financial reporting as of December 31, 2023, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon included therein, and incorporated herein by reference. Such consolidated financial statements and related financial statement schedule are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

S-12

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate by reference” information in this prospectus supplement or the accompanying prospectus that we have filed with the SEC. This means that we can disclose important information to you by referring you to another document filed separately with the SEC. You should review these filings as they may disclose a change in our business, prospects, financial condition or other affairs after the date of this prospectus supplement. The information incorporated by reference is considered to be part of this prospectus supplement and the accompanying prospectus, except for any information that is superseded by information that is included directly in this prospectus supplement or the accompanying prospectus relating to an offering of our securities.

We are incorporating by reference into this prospectus supplement the documents listed below and any additional documents that we may file with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date we file this prospectus supplement and before the termination of the offering of the securities made under this prospectus supplement. These additional documents include periodic reports, such as Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, and Current Reports on Form 8-K; except we are not incorporating by reference any information, documents or portions of any documents that are not deemed “filed” with the SEC, including any information furnished under Item 2.02 or Item 7.01 of any Current Report on Form 8-K or corresponding information furnished under Item 9.01 of Form 8-K (or included as an exhibit), in any past or future current report on Form 8-K that we may file with the SEC, unless specifically noted below.

This prospectus supplement incorporates by reference the documents listed below that we have filed with the SEC but have not been included or delivered with this document:

• Our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, filed with the SEC on February 9, 2024;

• The portions of our proxy statement for our 2023 annual meeting of stockholders filed with the SEC on March 27, 2023 that are incorporated by reference into our Annual Report on Form 10-K for the fiscal year ended December 31, 2022;

• Our Current Report on Form 8-K filed on February 15, 2024; and

• The description of our common stock contained in our Registration Statement on Form 8-A (registration no. 0-13480), filed with the SEC on November 2, 1987, as updated by the description of our common stock contained in Exhibit 4.1 to our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, and all amendments or reports filed for the purpose of updating such descriptions.

These documents contain important information about us and our financial condition. You may obtain a copy of any of these filings, or any of our future filings, from us without charge by requesting it in writing or by telephone at the following address or telephone number:

General Counsel

Arthur J. Gallagher & Co.

2850 Golf Road

Rolling Meadows, Illinois 60008-4050

These filings and reports can also be found on our investor relations website, located at https://investor.ajg.com. Additionally, you may obtain copies of any of these through the SEC’s website at http://www.sec.gov. The information on our website or any other website is not incorporated by reference in this prospectus supplement or the accompanying prospectus and should not be considered part of this prospectus supplement or the accompanying prospectus or any other filing we make with the SEC.

S-13

PROSPECTUS

DEBT SECURITIES

GUARANTEES

COMMON STOCK

PREFERRED STOCK

WARRANTS

DEPOSITARY SHARES

PURCHASE CONTRACTS

UNITS

We may from time to time offer to sell our debt securities, guarantees of debt securities, common stock or preferred stock, either separately or represented by warrants, depositary shares or purchase contracts, as well as units that include any of these securities or securities of other entities. The debt securities may consist of debentures, notes or other types of debt. We may also use this prospectus from time to time to register these securities that may be offered and sold by selling securityholders.

Each time we sell securities pursuant to this prospectus, we will provide a prospectus supplement and attach it to this prospectus. The prospectus supplement will contain more 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 these securities without a prospectus supplement describing the method and terms of the offering. You should carefully read this prospectus and any accompanying prospectus supplement, together with the information we incorporate by reference, before you make your investment decision.

We may sell these securities on a continuous or delayed basis directly, through agents or underwriters designated from time to time or through a combination of these methods. For additional information on the method of sale, you should refer to the section of this prospectus entitled “Plan of Distribution” on page 10. We reserve the sole right to accept, and together with any agents, dealers and underwriters, reserve the right to reject, in whole or in part, any proposed purchase of these securities. If any agents, dealers or underwriters are involved in the sale of these securities, the applicable prospectus supplement will set forth the names and any applicable commissions or discounts.

Our common stock is traded on the New York Stock Exchange under the symbol “AJG.”

Investing in these securities involves risks. You should carefully read and consider the risks referenced under “Risk Factors” beginning on page 3 of this prospectus for a discussion of the factors you should carefully consider, as well as the other information contained in or incorporated by reference in this prospectus or in any accompanying prospectus supplement, in determining whether to invest in these securities.

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 February 12, 2024.

TABLE OF CONTENTS

	Page
ABOUT THIS PROSPECTUS		1
ARTHUR J. GALLAGHER & CO.		2
RISK FACTORS		3
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS		4
USE OF PROCEEDS		8
SELLING SECURITYHOLDERS		9
DESCRIPTION OF SECURITIES		10
PLAN OF DISTRIBUTION		11
LEGAL MATTERS		12
EXPERTS		13
WHERE YOU CAN FIND MORE INFORMATION		14
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE		15

i

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, or SEC. Under the shelf registration process, we may from time to time, offer and sell the securities described in this prospectus or use this prospectus to register securities that may be offered and sold by selling securityholders from time to time. This prospectus provides a general description of the securities that we may offer and sell and that may be offered and sold by selling securityholders. Each time we offer and sell these securities, we will provide a prospectus supplement containing specific information about the terms of the securities being offered and the manner in which it may be offered. We may add, update or change the information contained in this prospectus by means of one or more prospectus supplements. 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. Before investing in our securities, both this prospectus and any prospectus supplement, together with the additional information described under the headings “Where You Can Find More Information” and “Incorporation of Certain Information by Reference,” should be carefully reviewed.

You should rely only on information contained in or incorporated by reference in this prospectus. We have not authorized anyone to give you any information or make any representation that is different from, or in addition to, that contained in this prospectus or in any of the materials that we have incorporated by reference into this document. If you are in a jurisdiction where offers to sell, or solicitations of offers to purchase, the securities offered by this document are unlawful, or if you are a person to whom it is unlawful to direct these types of activities, then the offer presented in this document does not extend to you. The information contained in this prospectus (including the information contained in any document incorporated by reference in this prospectus) speaks only as of the date of each such document, unless the information specifically indicates that another date applies.

1

ARTHUR J. GALLAGHER & CO.

We are engaged in providing insurance brokerage, reinsurance brokerage, consulting services, and third-party property/casualty claims settlement and administration services to entities and individuals in the U.S. and abroad.

We report our results in three segments: brokerage, risk management and corporate. Brokerage and risk management contributed approximately 86% and 14%, respectively, to 2023 revenues. We generate approximately 64% of our revenues for the combined brokerage and risk management segments domestically, with the remaining 36% generated internationally, primarily in the Australia, Canada, New Zealand and the U.K. The corporate segment did not generate revenues in 2023. Our ability to generate tax credits from qualified refined coal pursuant to Internal Revenue Code Section 45 ended in December 2021, and in 2022 we ran off existing chemical supplies as part of the wind down of such investments’ operations, which generated some revenues. Our major sources of operating revenues are commissions, fees and supplemental and contingent revenues from our brokerage operation, and fees including performance-based fees, from risk management operations.

For more information about our business, please refer to the “Business” section in our most recent Annual Report on Form 10-K filed with the SEC and incorporated by reference in this prospectus and the “Management’s Discussion and Analysis of Financial Condition and Results of Operations” sections of our most recent Annual Report on Form 10-K and all subsequent Quarterly Reports on Form 10-Q filed with the SEC and incorporated by reference in this prospectus.

Shares of our common stock are traded on the New York Stock Exchange under the symbol “AJG”. We were reincorporated as a Delaware corporation in 1972. Our executive offices are located at 2850 Golf Road, Rolling Meadows, Illinois 60008-4050, and our telephone number is (630) 773-3800.

Unless the context otherwise requires, the terms “we,” “our,” “Gallagher”, and “us” refer to Arthur J. Gallagher & Co. and/or its subsidiaries, as appropriate. The term “you” refers to a prospective investor.

2

RISK FACTORS

Investing in our securities involves risks. Before making an investment decision regarding our securities, in addition to the other information, documents or reports included in or incorporated by reference into this prospectus and any accompanying prospectus supplement or other offering materials, you should carefully consider the specific risks set forth under the caption “Risk Factors” in the applicable prospectus supplement as well as our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, which is incorporated by reference into this prospectus and any prospectus supplement, in their entirety, as the same may be amended, supplemented or superseded from time to time by our filings under the Exchange Act of 1934, as amended (the “Exchange Act”). For more information, see the section entitled “Where You Can Find More Information.” If any one or more of the risks set forth in the applicable prospectus supplement or documents incorporated by reference into this prospectus occur, they could materially and adversely affect our business, financial condition or results of operations. Consequently, the trading price of our securities could decline, and you may lose all or part of your investment.

3

CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This prospectus, the accompanying prospectus supplement and the documents they incorporate by reference, contain certain statements related to future results, or state our intentions, beliefs and expectations or predictions for the future, which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. Forward-looking statements relate to expectations or forecasts of future events. Such statements use words such as “anticipate,” “believe,” “estimate,” “expect,” “contemplate,” “forecast,” “project,” “intend,” “plan,” “potential,” and other similar terms, and future or conditional tense verbs like “could,” “may,” “might,” “see,” “should,” “will” and “would.” You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as: the impact of general economic conditions, including significant inflation, interest rates and market uncertainty; the effects of geopolitical volatility, including repercussions from the wars in Ukraine and the Middle East; market and industry conditions, including competitive and pricing trends; acquisition strategy including the expected size of our acquisition program; the expected impact of acquisitions and dispositions and integrating recent acquisitions, including comments regarding the expected benefits of our acquisition of the Willis Towers Watson plc treaty reinsurance brokerage operations (which we refer to as Willis Re), BCHR Holdings, L.P., and its subsidiaries, dba Buck (which we refer to as Buck), Cadence Insurance, Inc. (which we refer to as Cadence Insurance), Eastern Insurance Group, LLC (which we refer to as Eastern Insurance), My Plan Manager Group Pty Ltd (which we refer to as My Plan Manager), and other acquisitions larger than our typical tuck-in acquisitions and the expected duration and costs of integrating such large acquisitions; the development and performance of our services and products; changes in the composition or level of our revenues or earnings; our cost structure and the size and outcome of cost-saving or restructuring initiatives; future capital expenditures; future debt levels and anticipated actions to be taken in connection with maturing debt; future debt to earnings ratios; the outcome of contingencies; dividend policy; pension obligations; cash flow and liquidity; capital structure and financial losses; future actions by regulators; the outcome of existing regulatory actions, audits, reviews or litigation; the impact of changes in accounting rules; financial markets; interest rates; foreign exchange rates; matters relating to our operations; income taxes; expectations regarding our investments; human capital management, including diversity and inclusion initiatives, and environmental, social and governance matters, including climate-resilience and climate-advising products and services and carbon emissions. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors.

Potential factors that could impact results include:

• Global economic and geopolitical events, such as high inflation and related monetary policy responses including increased interest rates; a recession or economic downturn; failures of financial institutions and other counterparties or a potential United States (U.S.) government shutdown or gridlock over increasing the U.S. debt ceiling; political violence, and instability, including geo-economic fragmentation;

• Economic conditions that result in financial difficulties for underwriting enterprises or lead to reduced risk-taking capital capacity, for example, as a result of large payouts related to extreme weather events, or to the failure of such enterprises, including the increased risk of errors and omissions (which we refer to as E&O) claims against us;

• Risks that could negatively affect the success of our acquisition strategy, including the impact of current economic uncertainty on our ability to source, review and price acquisitions, continuing consolidation in our industry and interest in acquiring insurance brokers on the part of private equity firms and newly public insurance brokers, which makes it more difficult to identify targets and in some cases makes them more expensive, inaccurate assumptions and failure to realize expected benefits; the risk that we may not receive timely regulatory approval of pending transactions, closing risks; execution risks, integration risks, poor cultural fit, the risk of post-acquisition deterioration leading to

4

intangible asset impairment charges, and the risk we could incur or assume unanticipated liabilities such as cybersecurity issues or those relating to violations of anti-corruption and sanctions laws;

• Risks related to Willis Re, Buck, Cadence Insurance, Eastern Insurance, My Plan Manager and other acquisitions larger than our usual tuck-in acquisitions, including risks related to our ability to successfully integrate operations, the possibility that our assumptions may be inaccurate resulting in unforeseen obligations or liabilities and failure to realize the expected benefits of these acquisitions;

• Damage to our reputation, including as a result of environmental, social and governance (which we refer to as ESG) matters and the potential for the Internet and social media to magnify the effects of such reputational issues;

• Failure to meet our sustainability and ESG-related aspirations, goals and initiatives or to comply with increasingly complex climate-related regulations, including increased risks related to “greenwashing”;

• Emerging risks relating to the use of artificial intelligence (which we refer to as AI) in our business operations, including regulatory, data privacy and cybersecurity risks;

• Failure to apply technology, data analytics and AI effectively in driving value for our clients through technology-based solutions, or failure to gain internal efficiencies and effective internal controls through the application of technology and related tools;

• Risks associated with the use of AI in our business operations, including regulatory, data privacy, cybersecurity, E&O and competition risks;

• Failure to attract and retain experienced and qualified talent, including our senior management team, or adequately plan and execute for the succession of such leaders; increased costs resulting from increased compensation and benefits packages as a result of a tighter labor market, and negative effects from restrictions on non-competes at the state and federal level;

• Substantial increase in remote work among our employees, which may affect our corporate culture, productivity, collaboration and effective communication, increase cybersecurity or data breaches risks, heighten vulnerability to solicitations by competing firms and impact our ability to recruit and retain employees that prefer fully remote or fully-in-person work environments;

• A disaster or other significant disruption to business continuity for our own operations or those of third-parties on which we rely, including cybersecurity incidents; natural disasters; political violence and unrest in the U.S. or elsewhere around the world; for example, our substantial operations in India could be negatively impacted as a result of the dispute between India and Pakistan involving the Kashmir region, rising tensions between India and China, or incidents of terrorism in India, civil unrest or other reasons;

• Sustained increases in the cost of employee benefits and compensation expense;

• Risks arising from our international operations and changes in international conditions, including the risks posed by political and economic uncertainty in certain countries (including repercussions from the wars in Ukraine and the Middle East), risks related to maintaining regulatory and legal compliance across multiple jurisdictions (such as those relating to violations of anti-corruption, sanctions, protectionism, privacy laws and increasingly complex regulatory requirements related to climate change and sustainability issues), as well as, risks related to tariffs, trade wars, or climate change and other long-term environmental, social and governance matters and global health risks;

• Risks related to changes in U.S. or foreign tax laws, including a U.S. or foreign tax rate change, potential changes in guidance related to the U.S. Inflation Reduction Act, the Organisation for Economic Co-operation and Development’s (OECD) global minimum corporate tax regime, and other local policy changes;

5

• Competitive pressures, including as a result of innovation, in each of our businesses;

• Volatility or declines in premiums or other adverse trends in the insurance industry;

• The higher level of variability inherent in contingent and supplemental revenues versus standard commission revenues;

• Risks particular to our benefit consulting operations, including risks related to the acquisition of Buck;

• Risks particular to our third-party claims administrations operations, including risks related to the availability of RISX-FACS^®, our proprietary risk management information system, wage inflation, staffing shortages, any slowing of the trend toward outsourcing claims administration, and the concentration of large amounts of revenue with certain clients;

• Climate risks, including the risk of a systemic economic crisis and disruptions to our business caused by the transition to a low-carbon economy;

• Cyber-attacks or other cybersecurity incidents such as the ransomware incident we publicly disclosed in September 2020 and the heightened risk of such attacks as a result of the wars in Ukraine and the Middle East, improper disclosure of confidential, personal or proprietary data and changes to laws and regulations governing cybersecurity and data privacy;

• Unfavorable determinations related to contingencies and legal proceedings;

• Violations or alleged violations of the U.S. Foreign Corrupt Practices Act, the United Kingdom (U.K.) Bribery Act 2010 or other anti-corruption laws and the Foreign Account Tax Compliance provisions of the Hiring Incentives to Restore Employment Act, and the outcome of any existing or future investigation, review, regulatory action or litigation;

• Failure to comply with regulatory requirements, including those related to governance and control requirements in particular jurisdictions, international sanctions, including new sanctions laws as a result of the wars in Ukraine and the Middle East; laws relating to the disclosure of ESG-related matters; laws relating to the use of AI, or a change in regulations or enforcement policies that adversely affects our operations (for example, relating to insurance broker compensation methods or restrictions on non-competes);

• Changes to our financial presentation from new accounting estimates and assumptions;

• Intellectual property risks;

• Risks related to our legacy clean energy investments, including intellectual property claims, environmental and product liability claims, environmental compliance costs and the risk of disallowance by the Internal Revenue Service of previously claimed tax credits;

• The risk that our outstanding debt adversely affects our financial flexibility and restrictions and limitations in the agreements and instruments governing our debt;

• The risk of credit rating downgrades;

• The risk we may not be able to receive dividends or other distributions from subsidiaries, including the effects of significant changes in foreign exchange rates;

• The risk of share ownership dilution when we issue common stock; and

• Volatility of the price of our common stock.

Forward-looking statements are not guarantees of future performance. They involve risks, uncertainties and assumptions, including the risk factors referred to above. Our future performance and actual results or outcomes may differ materially from those expressed in forward-looking statements. Accordingly, you should not place

6

undue reliance on forward-looking statements, which speak only as of, and are based on information available to us on, the date of the applicable document. Many of the factors that will determine these results are beyond our ability to control or predict. All subsequent written and oral forward-looking statements attributable to us or any person acting on our behalf are expressly qualified in their entirety by the cautionary statements contained or referred to in this section. Forward-looking statements speak only as of the date that they are made, and we do not undertake any obligation to update any such statements or release publicly any revisions to these forward-looking statements to reflect events or circumstances after the date of this prospectus or to reflect new information, future or unexpected events or otherwise, except as required by applicable law or regulation. In addition, historical, current and forward-looking sustainability-related or ESG-related statements may be based on standards for measuring progress that are still developing, internal controls and processes that continue to evolve, and assumptions that are subject to change in the future.

A detailed discussion of the factors that could cause actual results to differ materially from our published expectations is contained under the heading “Risk Factors” in our filings with the SEC, including our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, and any reports we file with the SEC in the future, which are incorporated by reference into this prospectus.

7

USE OF PROCEEDS

Unless otherwise specified in a prospectus supplement accompanying this prospectus, the net proceeds from the sale of the securities to which this prospectus relates will be used for general corporate purposes. Net proceeds may be temporarily invested prior to use. We cannot predict whether the net proceeds, if invested, will yield a favorable return. Our management will have broad discretion in the application of the net proceeds we receive from the sale of the securities offered hereby, and you will be relying on the judgment of our management regarding the application of the net proceeds. We will not receive any proceeds from the sale of securities offered for sale in connection with this prospectus by selling securityholders. The selling securityholders will receive all of the net proceeds from those sales.

8

SELLING SECURITYHOLDERS

Information about selling securityholders, where applicable, will be set forth in a prospectus supplement, in a post-effective amendment, or in filings we make with the SEC under the Exchange Act, which are incorporated by reference herein.

9

DESCRIPTION OF SECURITIES

We will set forth in the applicable prospectus supplement a description of the debt securities, guarantees, common stock, preferred stock, warrants, depositary shares, purchase contracts, or units that may be offered under this prospectus.

10

PLAN OF DISTRIBUTION

We, or any selling securityholders, may offer and sell these securities to or through one or more underwriters, dealers and agents, or directly to purchasers, or a combination thereof, on a continuous or delayed basis. We will provide the specific plan of distribution for any securities to be offered in the applicable prospectus supplement.

11

LEGAL MATTERS

Gibson, Dunn & Crutcher LLP will pass upon the validity of the securities being offered by this prospectus. Any underwriters, dealers or agents will be represented by Cleary Gottlieb Steen & Hamilton LLP, New York, New York or by other legal counsel named in the applicable prospectus supplement.

12

EXPERTS

The consolidated financial statements and related financial statement schedule of Arthur J. Gallagher & Co. appearing in Arthur J. Gallagher & Co.’s Annual Report (Form 10-K) for the year ended December 31, 2023, and the effectiveness of Arthur J. Gallagher & Co.’s internal control over financial reporting as of December 31, 2023, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon included therein, and incorporated herein by reference. Such consolidated financial statements and related financial statement schedule are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

13

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC under the Exchange Act. You may access and read our SEC filings, including the complete registration statement of which this prospectus is a part and all of the exhibits to it, through commercial document retrieval services, which are available to the public, and also on the SEC’s website located at http://www.sec.gov. This site contains reports and other information that we file electronically with the SEC.

We have filed a registration statement, of which this prospectus is a part, and related exhibits with the SEC under the Securities Act. This prospectus, filed as part of the registration statement, does not contain all of the information set forth in the registration statement and its exhibits and schedules, portions of which have been omitted as permitted by the rules and regulations of the SEC. For further information about us and our securities, we refer you to the registration statement and its exhibits and schedules.

Statements in this prospectus about the contents of any contract, agreement or other documents are not necessarily complete and, in each instance, we refer you to the copy of such contract, agreement or document filed as an exhibit to the registration statement, with each such statement being qualified in all respects by reference to the document to which it refers.

14

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

We incorporate information into this prospectus by reference, which means that we can disclose important information to you by referring you to those documents, which have been separately filed with the SEC. The information incorporated by reference is considered to be part of this prospectus, and information that we file later with the SEC will automatically update and supersede this information. We are incorporating by reference our filings listed below and any additional documents that we may file with the SEC pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date of this prospectus and before the termination of the offering of the securities made under this prospectus; except we are not incorporating by reference any information, documents or portions of any documents that are not deemed “filed” with the SEC, including any information furnished under Item 2.02 or Item 7.01 of any Current Report on Form 8-K or corresponding information furnished under Item 9.01 of Form 8-K (or included as an exhibit) in any past or future current report on Form 8-K that we may file with the SEC, unless specifically noted below:

• Our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, filed with the SEC on February 9, 2024 (including the portions of our proxy statement for our 2024 annual meeting of stockholders incorporated by reference therein);

• The portions of our proxy statement for our 2023 annual meeting of stockholders filed with the SEC on March  27, 2023 that are incorporated by reference into our Annual Report on Form 10-K for the fiscal year ended December 31, 2022; and

• The description of our common stock contained in our Registration Statement on Form 8-A (registration no. 0-13480), filed with the SEC on November  2, 1987, as updated by the description of our common stock contained in Exhibit 4.1 to our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, and all amendments or reports filed for the purpose of updating such descriptions.

Information in this prospectus supersedes related information in the documents listed above and information in subsequently filed documents supersedes related information in both this prospectus and the incorporated documents.

You may request orally or in writing, and we will provide you with, a copy of these filings, at no cost, by calling us at (630) 773-3800 or by writing to us at the following address:

General Counsel

Arthur J. Gallagher & Co.

2850 Golf Road

Rolling Meadows, Illinois 60008-4050

These filings and reports can also be found on our investor relations website, located at https://investor.ajg.com. Additionally, you may obtain copies of any of these through the SEC’s website: http://www.sec.gov.

The information contained on our website does not constitute a part of this prospectus.

15

3,000,000 shares

Arthur J. Gallagher & Co.

Common Stock

PROSPECTUS SUPPLEMENT

Morgan Stanley

March 14, 2024