| Document And Entity Information | Nov. 12, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 12, 2021 |
| Entity Registrant Name | Cyteir Therapeutics, Inc. |
| Entity Central Index Key | 0001662244 |
| Entity Emerging Growth Company | true |
| Entity File Number | 001-40499 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 45-5429901 |
| Entity Address, Address Line One | 128 Spring St, Building A, Suite 510 |
| Entity Address, City or Town | Lexington |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02421 |
| City Area Code | 857 |
| Local Phone Number | 285-4140 |
| Entity Information, Former Legal or Registered Name | Not Applicable |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Ex Transition Period | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | CYT |
| Security Exchange Name | NONE |
