| Document and Entity Information | Feb. 05, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001701108 |
| Document Type | 8-K |
| Document Period End Date | Feb. 5, 2021 |
| Entity Registrant Name | SPERO THERAPEUTICS, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38266 |
| Entity Tax Identification Number | 46-4590683 |
| Entity Address, Address Line One | 675 Massachusetts Avenue |
| Entity Address, Address Line Two | 14th Floor |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02139 |
| City Area Code | (857) |
| Local Phone Number | 242-1600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.001 par value per share |
| Trading Symbol | SPRO |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
