Our Board of Directors (the “Board”) is soliciting your proxy to vote at the 20222023 Annual Meeting to be held via live webcast, on June 8, 2022,20, 2023, at 8:30 a.m., Eastern Time, and any adjournments or postponements of the meeting, which we refer to as the 2022 Annual Meeting.meeting. This proxy statement,Proxy Statement, along with the accompanying Notice of Annual Meeting of Stockholders, summarizes the purposes of the meeting and the information you need to know to vote at the 20222023 Annual Meeting.

You received the Notice because you owned shares of our common stock on April 26, 2022.25, 2023 (the “Record Date”).

As permitted by the rules of the U.S. Securities and Exchange Commission or the SEC,(the “SEC”) we may furnish our proxy materials to our stockholders by providing access to such documents on the Internet, rather than mailing printed copies of these materials to each stockholder. Stockholders will not receive printed copies of the proxy materials unless they request them. We believe that this process will expedite stockholders’ receipt of proxy materials, lower the costs of the 20222023 Annual Meeting and help to conserve natural resources. If you received the Notice by mail or electronically, you will not receive a printed or email copy of the proxy materials, unless you request one by following the instructions included in the Notice. Instead, the Notice instructs you as to how you may access and review all of the proxy materials and submit your proxy on the Internet. If you requested a paper copy of the proxy materials, you may authorize the voting of your shares by following the instructions on the proxy card, in addition to the other methods of voting described in this proxy statement.

Only stockholders of record at the close of business on the Record Date will be entitled to vote at the 20222023 Annual Meeting. On the Record Date, there were 20,535,62928,347,920 shares of common stock issued and outstanding. Our common stock is our only class of voting stock.

If you are a beneficial owner of shares registered in the name of your broker, bank, or other nominee, you should have received a voting instruction form with these proxy materials from that organization rather than from us. To vote prior to the 2023 Annual Meeting, follow the instructions provided by your broker, bank, or other nominee. If you are a beneficial owner, your deadline for submitting your vote before the Annual Meeting is 11:59 p.m., Tuesday, Eastern Time on Monday, June 7, 2022.19, 2023. If you wish to cast your vote during the 2023 Annual Meeting, you must obtain a Legal Proxy from your broker or other nominee and register with Computershare in advance of the meeting by following the instructions explained above. If you have already voted your shares and then obtain a Legal Proxy and register with Computershare, your original vote will be invalidated and you will be required to vote again during the Annual Meeting for your vote to be counted.

This Proxy Statement describes the proposals on which we would like you, as a stockholder, to vote at the 20222023 Annual Meeting. This Proxy Statement provides you with information on the proposals, as well as other information about us, so that you can make an informed decision as to whether and how to vote your stock.

At the 20222023 Annual Meeting, stockholders will act upon the following threefive proposals:

Our Board of Directors recommends that you vote as follows:

On each matter to be voted upon, you have one vote for each share of common stock you owned as of the Record Date.

If you are a stockholder of record and do not vote, your shares will not be voted.

If your shares are held by your broker, bank or other nominee, you should follow the instructions provided by your broker, bank or other nominee.

Our Bylaws provide that our business shall be managed by or under the direction of our Board of Directors. Our Board of Directors is divided into three classes for purposes of election. One class is elected at each annual meeting of stockholders to serve for a three-year term. OurThe size of our Board of Directors hashad been set at five members. Effective August 13, 2022, the currentBoard increased the size of the Board at fiveto six members and appointed Katherine Bach Kalin as a Class II director. The Board is currently classified into three classes as follows: (i) Jane Wasman and Robert Van Nostrand are members of Class I with a term ending at the 2023 Annual Meeting; (ii) David A. Scheinberg is a memberand Katherine Bach Kalin are members of Class II with a term ending at the 2024 Annual Meeting;annual meeting; and (iii) Angelos M. Stergiou and John Varian are members of Class III with a term ending at the 2022 Annual Meeting.2025 annual meeting.

Our Board of Directors, upon the recommendation of the Nominating and Corporate Governance Committee, has nominated each of Angelos M. StergiouRobert Van Nostrand and John VarianJane Wasman for election at the 20222023 Annual Meeting as a Class IIII director for a term of three years to serve until the 2025 Annual Meeting2026 annual meeting of stockholders, and until his successor hastheir successors have been elected and qualified.

Set forth below are the names of (i) the persons nominated for election as directors, and (ii) those directors whose terms do not expire this year, their ages, their offices in the Company, if any, their principal occupations or employment for at least the past five years, the length of their tenure as directors and the names of other public companies in which such persons hold or have held directorships during the past five years as of April 25, 2022.28, 2023. Additionally, information about the specific experience, qualifications, attributes or skills that led to our Board of Directors’ conclusion at the time of filing of this proxy statement that each person listed below should serve as a director is set forth below:

As required under Nasdaq listing standards, a majority of the members of a listed company’s board of directors must qualify as “independent,” as affirmatively determined by the board of directors of such listed company. Our Board consults with our internal and outside counsel to ensure that its determinations are consistent with relevant securities and other laws and regulations regarding the definition of “independent,” including those set forth in pertinent listing standards of Nasdaq, as in effect from time to time.

Consistent with these considerations, after review of all relevant identified transactions or relationships between each director, or any of his or her family members, and us, our senior management and our independent auditors, our Board has affirmatively determined that the following directors are independent directors within the meaning of the applicable Nasdaq listing standards: Jane Wasman, Robert L. Van Nostrand, each Class I directors, David A. Scheinberg, M.D., Ph.D., a and Katherine Bach Kalin, each Class II director,

Our Board has an independent Chair, Jane Wasman, who has served as Chair since 2017, and has authority, among other things, to preside over Board meetings and stockholder meetings, and shall have such powers and duties as may from time to time be assigned by the Board. Accordingly, the Chair has substantial ability to shape the work of the Board.

Our Board reviews the Nasdaq listing standards’ definition of “independence” for Audit Committee members on an annual basis and has determined that all members of our Audit Committee are independent (as currently defined in Rule 5605(c)(2)(A)(i) and (ii) of the Nasdaq listing standards and under Rule 10A-3 under the Exchange Act).

Our Board has also determined that Messrs. Varian and Van Nostrand each qualify as an “audit committee financial expert,” as defined in applicable SEC rules. Our Board made a qualitative assessment of the level of knowledge and experience of both Mr. Varian and Mr. Van Nostrand based on a number of factors, including the formal education and experience of each of Messrs. Van Nostrand and Varian as former chief financial officers for public reporting companies, and the status of Messrs.Mr. Van Nostrand as a Certified Public Accountant and Mr. Varian as a former Certified Public Accountant.

The Compensation Committee is currently comprised of four directors: Ms. Wasman, Dr. Scheinberg, Mr. Van Nostrand and VarianMs. Kalin. Each of Ms. Wasman, Mr. Van Nostrand, Dr. Scheinberg and Ms. Kalin are independent (as independence is currently defined in Rule 5605(d)(2) of the Nasdaq listing standards). Dr. Scheinberg was appointed to the Compensation Committee effective January 1, 2022 and Ms. Kalin was appointed effective October 13, 2022. The Compensation Committee met three times in 2022 and took action by written consent three times in 2022. Our Board has adopted a written Compensation Committee charter that is available to stockholders in the corporate governance section of our website at: https://www.sellaslifesciences.com/investors/corporate-governance/default.aspx#section=documents.

The Audit Committee, which consists entirely of directors who meet the independenceNominating and experience requirements of the Nasdaq Capital Market, has furnished the following report:

Messrs. John VarianThe Nominating and Robert L. Van Nostrand, and Ms. Jane Wasman.

Each year, our CompensationOur Nominating and Corporate Governance Committee reviews with management our executive compensation tables and accompanying narrative disclosure and considers whether to recommendbelieves that it be includedis in proxythe best position to identify, review, evaluate and select qualified candidates for Board membership, based on the comprehensive criteria for Board membership approved by the Board. Generally, our Nominating and Corporate Governance Committee will consider candidates recommended from several sources, such as other directors or officers, stockholders, third party search firms or other appropriate sources. Once identified, the Nominating and Corporate Governance Committee will evaluate a candidate’s qualifications. The Nominating and Corporate Governance Committee believes that candidates for director should have certain qualifications, including the ability to read and understand basic financial statements and other filings.

In the case of incumbent directors whose terms of office are set to expire, our Nominating and Corporate Governance Committee reviews these directors’ overall service to the Company during their terms, including the number of meetings attended, level of participation, quality of performance and any other relationships and transactions that Radford’s work formight impair the Compensationdirectors’ independence. The Committee also takes into account the results of the Board’s self-evaluation, conducted annually on a group and individual basis.

Although the Nominating and Corporate Governance Committee does not raise any conflict of interest. On an annual basis, Radford reviews our executive officer and director compensation relative tohave a peer group and against survey data available to Radford. Workingformal policy with Radford, the Compensation Committee reviews and adjusts our peer group in the latter part of each year for upcoming end-of-year compensation decisions. Decisions regarding 2022 salaries and short term non-equity incentive plan compensation were made by our Compensation Committee in early 2022 by referenceregard to the peer group determinedconsideration of director candidates recommended by stockholders, it has the power and authority to consider recommendations for Board nominees and proposals submitted by the Company’s stockholders.

If a stockholder wishes to propose a candidate for consideration as a nominee for election to our Compensation CommitteeBoard of Directors, that stockholder must follow the procedures described in our Bylaws. Any such recommendation should be made in writing on a timely basis as set forth in our Bylaws to our Corporate Secretary at the end of 2021. In late 2021, the Compensation Committeeour principal office and Radford, using industry standard parameters for establishing a peer group, sought to identify companies that fitshould be accompanied by the following criteria:information concerning the proposed nominee:

The Nominating and Corporate Governance Committee is responsible for identifying, reviewing and evaluating candidates to serve as directors of the Company (consistent with criteria approved by the Board), reviewing and evaluating incumbent directors, selecting candidates for election to our Board, making recommendations to our Board regarding the membership of the committees of the Board, and assessing the performance of management and our Board.

Our Nominating and Corporate Governance Committee believes that it is in the best position to identify, review, evaluate and select qualified candidates for Board membership, based on the comprehensive criteria for Board membership approved by the Board. Generally, our Nominating and Corporate Governance Committee will consider candidates recommended from several sources, such as other directors or officers, stockholders, third party search firms or other appropriate sources. Once identified, the Nominating and Corporate Governance Committee will evaluate a candidate’s qualifications. The Nominating and Corporate Governance Committee believes that candidates for director should have certain qualifications, including the ability to read and understand basic financial statements and the possession of the highest personal integrity and ethics. Candidates for director should also be over 21 years of age. Our Nominating and Corporate Governance Committee also intends to considerconsiders such factors as possessing relevant expertise enabling the candidate to offer advice and guidance to management, having sufficient time to devote to the affairs of the Company, demonstrated excellence in his or her field, having the ability to exercise sound business judgment and having the commitment to rigorously represent the long-term interests of our stockholders. However, our Nominating and Corporate Governance Committee retains the right to modify these qualifications from time to time. Candidates for director nominees are reviewed in the context of the current composition of the Board, the operating requirements of the Company and the long-term interests of stockholders. In conducting this assessment, our Nominating and Corporate Governance Committee typically considers diversity, age, skills and such other factors as it deems appropriate, given our current needs and the needs of our Board, to maintain a balance of knowledge, experience and capability. The Nominatingcapability and Corporate Governance Committee considers issues of diversity among its members in identifying and considering nominees for director, and strivesstriving where appropriate to achieve a diverse balance of backgrounds, perspectives, experience, age, gender, ethnicity and country of citizenship on our Board of Directors and its committees. Our Nominating and Corporate Governance Committee also determines whether the nominee is independent for Nasdaq purposes, which determination is based upon applicable Nasdaq listing standards, applicable SEC rules and regulations and the advice of counsel, if necessary.

In the case of incumbent directors whose terms of office are set to expire, our Nominating and Corporate Governance Committee reviews these directors’ overall service to the Company during their terms, including the number of meetings attended, level of participation, quality of performance and any other relationships and transactions that might impair the directors’ independence. The Committee also takes into account the results of the Board’s self-evaluation, conducted annually on a group and individual basis. In the case of new director candidates, our Nominating and Corporate Governance Committee also determines whether the nominee is independent for Nasdaq purposes, which determination is based upon applicable Nasdaq listing standards, applicable SEC rules and regulations and the advice of counsel, if necessary. The Nominating and Corporate Governance Committee then uses its network of contacts to compile a list of potential candidates, but may also engage, if it deems appropriate, a professional search firm. The Nominating and Corporate Governance Committee conducts any appropriate and necessary inquiries into the backgrounds and qualifications of possible candidates after considering the function and needs of the Board.

Our Nominating and Corporate Governance Committee believes that it is in the best position to identify, review, evaluate and select qualified candidates for Board membership, based on the comprehensive criteria for Board membership approved by the Board. The Nominating and Corporate Governance Committee meets to discuss and consider the candidates’ qualifications and then selects a nominee for recommendation to the Board by majority vote. Although the Nominating and Corporate Governance Committee does not have a formal policy with regard to the consideration of director candidates recommended

The Board expects that the views of our stockholders will be heard by the Board, its committees or individual directors, as applicable, and that appropriate responses be provided to stockholders on a timely basis. Stockholders wishing to formally communicate with the Board, any committee of the Board, the independent directors as a group or any individual director may send communications directly to us at SELLAS Life Sciences Group, Inc., Times Square Tower, 7 Times Square, Suite 2503, New York, New York 10036, Attention: Corporate Secretary. All clearly marked written communications, other than unsolicited advertising or promotional materials, are logged and copied, and forwarded to the director(s) to whom the communication was addressed. Undirected communications will be distributed to our entire Board, or to any individual director or directors as appropriate, depending on the facts and circumstances outlined in the communications. Please note that the foregoing communication procedure does not apply to (i) stockholder proposals pursuant to Exchange Act Rule 14a-8 and communications made in connection with such proposals or (ii) service of process or any other notice in a legal proceeding.

We have adopted a Code of Business Conduct and Ethics or the Code,(the “Code”) which, along with our Amended and Restated Certificate of Incorporation, Amended and Restated Bylaws and the charters of our Board committees, provides a framework for the governance of our company. The Board’s Nominating and

The Code reflects current best practices and enhances our personnel’s understanding of our standards of ethical business practices, promotes awareness of ethical issues that may be encountered in carrying out an employee’s or director’s responsibilities, and provides clarity as to how to address ethical issues that may arise.

Annual bonusesFor 2021 and 2022, short-term incentive cash compensation for our executive officers arewas determined based on the achievement of performance metrics related to our corporate goals typically comprised of a mix of clinical, financial and business development performance objectives.goals. The corporate goals are proposed by senior management and reviewed and approved by theour Compensation Committee and our Board of Directors on an annual basis at the start of each year. Dr. Stergiou’s bonus is based 100%Our annual corporate goals are typically focused on the level of achievement of specified clinical development goals with a focus on advancing our clinical pipeline. The annual goals also usually include the pursuit of certain corporate development goals, such as business development initiatives and achievement of cash management strategies in order to assure adequate funding for our clinical development programs. The goals are designed to require significant effort in order to achieve our clinical development and operational goals while also being achievable with the bonuses of the other executive officers are computed based on a combination of achievement of corporate goalsrequisite hard work and individual performance.dedication. Awards made under our short-term cash incentive program utilize a pay-for-performance strategy and compensateaward our NEOs for the level of achievement of specified annual corporate goals, as well as individual goals for our NEOs other than our CEO whose short-term incentive compensation is based 100% on the level of achievement of our corporate goals. Our annual corporate goals are focused on the achievement of meaningful and specified clinical development goals with a focus on advancing our clinical pipeline. The annual goals also usually include the pursuit of certain corporate development actions, such as business development initiatives and achievement of cash management strategies in order to assure adequate funding for our clinical development programs and cash run-way. The annual goals are proposed by senior management and reviewed and approved by our Compensation Committee and our Board. The goals are designed to require significant effort in order to achieve our clinical development and operational goals while also being achievable with the requisite hard work and dedication.

In March 2021, the Compensation Committee recommended, and our Board approved, the grant of 118,500 47,250 and 47,250 options to Dr. Stergiou and Ms. Wood, and Dr. Cicic, respectively, at an exercise price of $8.00 per