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Cover - USD ($)
 12 Months Ended
Dec. 31, 2021
Mar. 30, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2021    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-33958    
Entity Registrant Name SELLAS Life Sciences Group, Inc.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 20-8099512    
Entity Address, Address Line One 7 Times Square    
Entity Address, Address Line Two Suite 2503    
Entity Address, City or Town New York    
Entity Address, State or Province NY    
Entity Address, Postal Zip Code 10036    
City Area Code (646)    
Local Phone Number 200-5278    
Title of 12(b) Security Common Stock, $0.0001 Par Value per share    
Trading Symbol SLS    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Public Float     $ 176,103,964
Entity Common Stock, Shares Outstanding   15,905,999  
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 2022 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.    
Entity Central Index Key 0001390478    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Amendment Flag false