8-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF THE

SECURITIES EXCHANGE ACT OF 1934

Date of report (Date of earliest event reported): June 17, 2025

SELLAS

Life Sciences Group, Inc.

(Exact name of registrant as specified in its charter)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligations of the registrant under any of the following provisions:

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ¨

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. ¨

Item 5.07 Submission of Matters to a Vote of Security Holders.

Annual Meeting of Stockholders

On June 17, 2025, SELLAS Life Sciences Group, Inc. (the “Company”) held its Annual Meeting of Stockholders (“Annual Meeting”). There were 61,319,182 shares of common stock, or approximately 64.85% of all outstanding shares, present in person or represented by proxy. At the Annual Meeting, the stockholders voted on the following three proposals and cast their votes as described below.

Proposal 1

The Company’s stockholders re-elected each of the following nominees as a Class III director to the Board of the Company to hold office until the 2028 Annual Meeting of Stockholders and until their respective successors have been duly elected and qualified.

Proposal 2

The Company’s stockholders ratified the appointment of Baker Tilly US, LLP (as the successor to Moss Adams LLP) as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2025.

Proposal 3

The Company’s stockholders approved an amendment to the Company’s 2021 Employee Stock Purchase Plan (“ESPP”) to increase the number of shares of common stock available for sale under the ESPP by 800,000.

Proposal 4

The Company’s stockholders approved the non-binding, advisory resolution on the executive compensation of the Company’s named executive officers.

Proposal 5

The Company’s stockholders recommended, on a non-binding advisory basis, the frequency of future advisory votes on the compensation of the Company’s named executive officers.

Proposal 6

The Company’s stockholders approved an adjournment of the Annual Meeting, if necessary, to solicit additional proxies in favor of proposals 1, 2, 3, 4 and 5.

No other matters were submitted to or voted on by the Company’s stockholders at the Annual Meeting.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.