Exhibit 99.1 SELLAS Life Sciences Reports Full Year 2023 Financial Results and Provides Corporate Update – Announced Phase 2a study of SLS009 in r/r AML Showing 50% Response Rate in the Selected Optimal Dose of 30 mg BIW Exceeding the Targeted 20% and 100% Response Rate in Patients with Identified Biomarkers - – Completed Enrollment in Phase 3 REGAL Study of Galinpepimut-S in Patients with Acute Myeloid Leukemia; Steering Committee Guided Interim Analysis May Be Imminent; IDMC Scheduled in Late April- – Pha ...

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-K ________________________________ (Mark One) ☒ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2023 or ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from _________ to _________ Commission File Number: 001-33958 SELLAS Life Sciences G ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-Q ________________________________ ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended September 30, 2023 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number: 001-33958 SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter) Delaware ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-Q ________________________________ ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended June 30, 2023 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number: 001-33958 SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter) Delaware 20-8 ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-Q ________________________________ ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2023 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number: 001-33958 SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter) Delaware 20- ...

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-K ________________________________ (Mark One) ☒ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2022 or ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from _________ to _________ Commission File Number: 001-33958 SELLAS Life Sciences G ...