| Document and Entity Information | Nov. 15, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 15, 2021 |
| Entity File Number | 001-40869 |
| Entity Registrant Name | Theseus Pharmaceuticals, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 83-0712806 |
| Entity Address State Or Province | MA |
| Entity Address, Address Line One | 245 Main Street |
| Entity Address, City or Town | Cambridge |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | 857 |
| Local Phone Number | 400-9491 |
| Title of 12(b) Security | Common stock, par value $0.0001 per share |
| Trading Symbol | THRX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001745020 |
| Amendment Flag | false |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
