SELLAS Life Sciences Group, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-K

(Mark One)

For the fiscal year ended December 31, 2024

or

For the transition period from _________ to _________

Commission File Number:001-33958

SELLAS Life Sciences Group, Inc.

(Exact name of registrant as specified in its charter)

7 Times Square, Suite 2503, New York, NY 10036

(Address of principal executive officers)

(646) 200-5278

(Registrant's telephone number, including area code)

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yeso No x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Exchange Act. Yeso No x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the Registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes x Noo

Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). Yes x Noo

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and "emerging growth company" in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. o

Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act (15 U.S.C. 7262(b)) by the registered public accounting firm that prepared or issued its audit report.☐

If securities are registered pursuant to Section 12(b) of the Act, indicate by check mark whether the financial statements of the registrant included in the filing reflect the correction of an error to previously issued financial statements.☐

Indicate by check mark whether any of those error corrections are restatements that required a recovery analysis of incentive-based compensation received by any of the registrant’s executive officers during the relevant recovery period pursuant to §240.10D-1(b). ☐

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).☐ Yesx No

The aggregate market value of the registrant's common stock, $0.0001 per value per share, held by non-affiliates of the registrant on June 30, 2024, the last business day of the registrant's most recently completed second fiscal quarter, was $68,573,913 (based on the closing sales price of the registrant's common stock on that date). Shares of the registrant's common stock held by each officer and director and each person who owns 5% or more of the outstanding common stock of the registrant have been excluded in that such persons may be deemed to be affiliates. This determination of affiliate status is not necessarily a conclusive determination for other purposes. As of March 19, 2025, SELLAS Life Sciences Group, Inc. had outstanding 90,846,692 shares of common stock, $0.0001 par value per share, exclusive of treasury shares.

DOCUMENTS INCORPORATED BY REFERENCE

Certain information required in Part III of this Annual Report on Form 10-K is incorporated from the registrant’s Proxy Statement for its 2025 Annual Meeting of Stockholders to be filed with the Securities and Exchange Commission pursuant to Regulation 14A not later than 120 days after the end of the fiscal year covered by this Form 10-K, provided that if such Proxy Statement is not filed within such period, such information will be included in an amendment to this Form 10-K to be filed within such 120-day period.

SPECIAL NOTE REGARDING FORWARD LOOKING STATEMENTS

This Annual Report on Form 10-K includes forward-looking statements that reflect our current views with respect to our development programs, business strategy, business plan, financial performance and other future events. These statements include forward-looking statements both with respect to us, specifically, and our industry, in general. Such forward-looking statements include the words "expect," "intend,” "plan," "believe," "project," "estimate,” "may,” "should," "anticipate," "will" and similar statements of a future or forward-looking nature identify forward-looking statements and include, without limitation, statements regarding:

•our future financial and business performance;

•strategic plans for our business and product candidates;

•our ability to develop or commercialize products;

•the expected results and timing of clinical trials and nonclinical studies;

•our ability to comply with the terms of our license agreements;

•developments and projections relating to our competitors and industry;

•our expectations regarding our ability to obtain, develop and maintain intellectual property protection and not infringe on the rights of others;

•our ability to retain and attract highly-skilled executive officers and employees;

•our future capital requirements and the timing of those requirements and sources and uses of cash;

•our ability to obtain funding for our operations; and

•changes in applicable laws or regulations.

These statements are subject to known and unknown risks, uncertainties and assumptions that could cause actual results to differ materially from those projected or otherwise implied by the forward-looking statements, including the following:

•risks associated with preclinical or clinical development and trials;

•changes in the assumptions underlying our expectations regarding our future business or business model;

•our ability to develop, manufacture and commercialize product candidates;

•general economic, financial, legal, political and business conditions and changes in domestic and foreign markets;

•changes in applicable laws or regulations;

•the impact of natural disasters, including climate change, and the impact of health epidemics, on our business;

•the size and growth potential of the markets for our products, and our ability to serve those markets;

•market acceptance of our planned products;

•our ability to raise capital;

•the possibility that we may be adversely affected by other economic, business, and/or competitive factors; and

•other risks and uncertainties set forth in this report in the section entitled “Risk Factors.”

Forward-looking statements are neither historical facts nor assurances of future performance. Instead, they are based only on our current beliefs, expectations and assumptions regarding the future of our business, future plans and strategies, projections, anticipated events and trends, the economy and other future conditions. Forward-looking statements are subject to inherent uncertainties, risks and changes in circumstances that are difficult to predict and many of which are outside of our control. Given these risks and uncertainties, you should not place undue reliance on these forward-looking statements. We undertake no obligation to publicly update or review any forward-looking statement, whether as a result of new information, future developments or otherwise, except as required by law.