| Document and Entity Information | Nov. 09, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001789972 |
| Document Type | 8-K |
| Document Period End Date | Nov. 9, 2021 |
| Entity Registrant Name | CULLINAN ONCOLOGY, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-39856 |
| Entity Tax Identification Number | 81-3879991 |
| Entity Address, Address Line One | Cullinan Oncology, Inc. |
| Entity Address, Address Line Two | One Main Street |
| Entity Address, Address Line Three | Suite 520 |
| Entity Address, City or Town | Cambridge |
| Entity Address, Country | MA |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | (617) |
| Local Phone Number | 410-4650 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.0001 par value per share |
| Trading Symbol | CGEM |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
