| Document and Entity Information | Oct. 20, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001750019 |
| Document Type | 8-K |
| Document Period End Date | Oct. 20, 2021 |
| Entity Registrant Name | TCR2 THERAPEUTICS INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38811 |
| Entity Tax Identification Number | 47-4152751 |
| Entity Address, Address Line One | 100 Binney Street |
| Entity Address, Address Line Two | Suite 710 |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | 617 |
| Local Phone Number | 949-5200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | TCRR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
