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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): January 7, 2026

SELLAS

Life Sciences Group, Inc.

(Exact name of registrant as specified in its charter)

Delaware

001-33958

20-8099512

(State or other jurisdiction of incorporation or organization)

(Commission

File Number)

(I.R.S. Employer

Identification No.)

7 Times Square, Suite 2503 New York, NY 10036

(Address of Principal Executive Offices) (Zip Code)

Registrant’s telephone number, including area code: (646) 200-5278

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:

Title of each class Trading

symbol(s) Name of each exchange on which

registered

Common Stock, $0.0001 par value per share SLS The Nasdaq Stock Market LLC

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 2.02.Results of Operations and Financial Condition.

As of December 31, 2025, SELLAS Life Sciences Group, Inc. (the “Company”) estimates that its unaudited cash and cash equivalents position was $71.8 million. This amount is unaudited and preliminary and is subject to the completion of financial closing procedures, including management’s reviews. As a result, this amount may differ materially from the amount that will be reflected in the Company’s financial statements as of and for the year ended December 31, 2025.

Item 8.01Other Events

Subsequent to December 31, 2025, the Company received approximately $26.5 million in proceeds from the exercise of common stock warrants.

As of January 7, 2026, the Company had 170,282,026 shares of common stock outstanding.

2

Item 9.01.Financial Statements and Exhibits.

(d)       Exhibits

Exhibit Number

Description

104

Cover Page Interactive Data File (embedded within the Inline XBRL document)

3

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.

SELLAS Life Sciences Group, Inc.

Date: January 8, 2026 By: /s/ John T. Burns

Name: John T. Burns

Title: Senior Vice President, Chief Financial Officer

sls-202511120001390478FALSE00013904782025-11-122025-11-12

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): November 12, 2025

SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter)

Delaware001-3395820-8099512 (State or other jurisdiction of incorporation or organization)(Commission File Number) (I.R.S. Employer Identification No.)

7 Times Square, Suite 2503 New York, NY 10036

(Address of Principal Executive Offices) (Zip Code)

Registrant’s telephone number, including area code: (646) 200-5278

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:

Title of each classTrading symbol(s)Name of each exchange on which registered Common Stock, $0.0001 par value per shareSLSThe Nasdaq Stock Market LLC

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 2.02. Results of Operations and Financial Condition.

On November 12, 2025, SELLAS Life Sciences Group, Inc. (“SELLAS”) issued a press release (the “Press Release”) announcing its financial results for the quarter ended September 30, 2025 and providing a corporate update.

A copy of the Press Release is furnished hereto as Exhibit 99.1 and is incorporated by reference herein. The information contained in this Item 2.02 and in the Press Release furnished as Exhibit 99.1 to this Current Report on Form 8-K shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended, or otherwise subject to the liabilities of that Section or Sections 11 and 12(a)(2) of the Securities Act of 1933, as amended. The information contained in this Item 2.02 and in the Press Release furnished as Exhibit 99.1 to this Current Report on Form 8-K shall not be incorporated by reference into any filing with the SEC made by SELLAS whether made before or after the date hereof, regardless of any general incorporation language in such filing.

Item 9.01.    Financial Statements and Exhibits.

(d)    Exhibits

Exhibit NumberDescription 99.1Press Release dated November 12, 2025

104Cover Page Interactive Data File (embedded within the Inline XBRL document)

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.

SELLAS Life Sciences Group, Inc.

Date: November 12, 2025  By: /s/ John T. Burns Name:John T. Burns Title:Senior Vice President, Chief Financial Officer

sls-202508120001390478FALSE00013904782025-08-122025-08-12

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): August 12, 2025

SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter)

Delaware001-3395820-8099512 (State or other jurisdiction of incorporation or organization)(Commission File Number) (I.R.S. Employer Identification No.)

7 Times Square, Suite 2503 New York, NY 10036

(Address of Principal Executive Offices) (Zip Code)

Registrant’s telephone number, including area code: (646) 200-5278

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:

Title of each classTrading symbol(s)Name of each exchange on which registered Common Stock, $0.0001 par value per shareSLSThe Nasdaq Stock Market LLC

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 2.02. Results of Operations and Financial Condition.

On August 12, 2025, SELLAS Life Sciences Group, Inc. (“SELLAS”) issued a press release (the “Press Release”) announcing its financial results for the quarter ended June 30, 2025 and providing a corporate update.

A copy of the Press Release is furnished hereto as Exhibit 99.1 and is incorporated by reference herein. The information contained in this Item 2.02 and in the Press Release furnished as Exhibit 99.1 to this Current Report on Form 8-K shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended, or otherwise subject to the liabilities of that Section or Sections 11 and 12(a)(2) of the Securities Act of 1933, as amended. The information contained in this Item 2.02 and in the Press Release furnished as Exhibit 99.1 to this Current Report on Form 8-K shall not be incorporated by reference into any filing with the SEC made by SELLAS whether made before or after the date hereof, regardless of any general incorporation language in such filing.

Item 9.01.    Financial Statements and Exhibits.

(d)    Exhibits

Exhibit NumberDescription 99.1Press Release dated August 12, 2025

104Cover Page Interactive Data File (embedded within the Inline XBRL document)

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.

SELLAS Life Sciences Group, Inc.

Date: August 12, 2025  By: /s/ John T. Burns Name:John T. Burns Title:Senior Vice President, Chief Financial Officer