| Document and Entity Information | Nov. 09, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001829802 |
| Document Type | 8-K |
| Document Period End Date | Nov. 9, 2021 |
| Entity Registrant Name | Sensei Biotherapeutics, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-39980 |
| Entity Tax Identification Number | 83-1863385 |
| Entity Address, Address Line One | 1405 Research Blvd |
| Entity Address, Address Line Two | Suite 125 |
| Entity Address, City or Town | Rockville |
| Entity Address, State or Province | MD |
| Entity Address, Postal Zip Code | 20850 |
| Local Phone Number | 243-8000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | SNSE |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
