| Cover | Sep. 23, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 23, 2021 |
| Entity File Number | 001-33958 |
| Entity Registrant Name | SELLAS Life Sciences Group, Inc. |
| Entity Central Index Key | 0001390478 |
| Entity Tax Identification Number | 20-8099512 |
| Entity Incorporation, State or Country Code | DE |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.0001 par value per share |
| Trading Symbol | SLS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
