SELLAS Life Sciences Group, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

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SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a)

of the Securities Exchange Act of 1934

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Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

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Preliminary Proxy Statement

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Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

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Definitive Proxy Statement

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Definitive Additional Materials

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Soliciting Material Pursuant to § 240.14a-12

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(Name of Registrant as Specified In Its Charter)

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(Name of Person(s) Filing Proxy Statement if Other Than the Registrant)

Payment of Filing Fee (Check the appropriate box):

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No fee required.

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Fee previously paid with preliminary materials.

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Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11.

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April 23, 2026​

To Our Stockholders:

You are cordially invited to attend the 2026 Annual Meeting of Stockholders of SELLAS Life Sciences Group, Inc. (the “Company”) to be held at 8:30 a.m. Eastern Time on June 16, 2026 (the “2026 Annual Meeting”), unless postponed or adjourned to a later date. We will hold the 2026 Annual Meeting virtually as a live webcast at www.virtualshareholdermeeting.com/SLS2026. You will not be able to attend the 2026 Annual Meeting in person. Please refer to the proxy materials for more information on how to access the 2026 Annual Meeting.

The enclosed Notice of Annual Meeting of Stockholders sets forth the proposals that will be presented at the 2026 Annual Meeting, which are described in more detail in the enclosed Proxy Statement. Our Board of Directors recommends that you vote “FOR” Proposals 1, 2, 3, 4, and 5 as set forth in the Proxy Statement.

Under Securities and Exchange Commission rules that allow companies to furnish proxy materials to stockholders over the Internet instead of a printed copy of proxy materials, we have elected to deliver our proxy materials to our stockholders over the Internet. This delivery process allows us to provide stockholders with the information they need, while at the same time conserving natural resources and lowering the cost of delivery. On or about May 1, 2026, we expect to mail to our stockholders a Notice of Internet Availability of Proxy Materials (the “Notice”), which contains instructions on how to access our Proxy Statement and our Annual Report on Form 10-K for the year ended December 31, 2025. The Notice will also provide instructions on how to vote via the Internet as well as by telephone or mail.

We hope you will be able to attend the 2026 Annual Meeting. Whether you plan to attend the 2026 Annual Meeting or not, it is important that you cast your vote. When you have finished reading the Proxy Statement, we urge you to submit your vote so that your shares will be represented and voted at the 2026 Annual Meeting, whether or not you can attend.

On behalf of the Board and the management team, we thank you for your ongoing support of, and continued interest in, SELLAS Life Sciences Group, Inc.

Times Square Tower, 7 Times Square, Suite 2503

New York, New York 10036

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

To the Stockholders of SELLAS Life Sciences Group, Inc.:

You are invited to attend the 2026 Annual Meeting of Stockholders (“2026 Annual Meeting”) of SELLAS Life Sciences Group, Inc. (“SELLAS,” “we,” “us,” “our,” or “Company”) on June 16, 2026, at 8:30 a.m. Eastern Time. The 2026 Annual Meeting will be held exclusively online as a live webcast at www.virtualshareholdermeeting.com/SLS2026. There is no physical location for the 2026 Annual Meeting.

The 2026 Annual Meeting will be held for the following purposes, which are more fully described in the proxy statement accompanying this notice:

To elect two (2) Class I directors to serve on our Board of Directors for a three-year term expiring on the date on which our annual meeting of stockholders is held in 2029;

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To ratify the appointment by our Audit Committee of Baker Tilly US, LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026;

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To approve an amendment to the Company’s 2023 Amended and Restated Equity Incentive Plan (the “2023 Equity Plan”) to increase the number of shares of Common Stock authorized for issuance under the 2023 Equity Plan by 20,000,000 shares;

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To approve, on a non-binding advisory basis, the compensation of the Company’s named executive officers; and

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To approve any postponement or adjournment of the 2026 Annual Meeting, from time to time, if necessary, to solicit additional proxies if there are not sufficient votes at the time of the 2026 Annual Meeting to adopt the proposals set forth above or to establish a quorum.

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In addition, stockholders may be asked to consider and vote upon such other business as may be properly brought before the meeting and any adjournment or postponement thereof.

Only stockholders of record of SELLAS common stock at the close of business on April 21, 2026, the record date, are entitled to notice of and to vote at the 2026 Annual Meeting and at any adjournments or postponements thereof. A list of stockholders of record will be available at the 2026 Annual Meeting and, during the 10 days prior to the annual meeting, at our principal executive offices located at Times Square Tower, 7 Times Square, Suite 2503, New York, New York 10036.

Important Notice Regarding Internet Availability of Proxy Materials for the Annual Meeting to be Held on June 16, 2026: SELLAS’ Proxy Statement and our Annual Report on Form 10-K for the year ended December 31, 2025 (the “2026 Annual Report”) are available at www.proxyvote.com. To access these proxy materials, please follow the instructions found on your Notice of Internet Availability of Proxy Materials (the “Notice”) or proxy card.

Your vote is important regardless of the number of shares of common stock you own. Whether or not you expect to attend the 2026 Annual Meeting, please submit your proxy by Internet, telephone, or mail by following the instructions found on the Notice or proxy card so that your shares of common stock may be represented and voted at the 2026 Annual Meeting.

Regardless of the number of shares of common stock of the Company that you own, your vote is important. Thank you for your continued support, interest and investment in the Company.

By Order of the Board of Directors,

Stacy E. Yeung

Vice President, General Counsel &

Corporate Secretary

New York, New York

April 23, 2026