| Document and Entity Information | Mar. 10, 2023 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001785173 |
| Document Type | 8-K |
| Document Period End Date | Mar. 10, 2023 |
| Entity Registrant Name | 89bio, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-39122 |
| Entity Tax Identification Number | 36-4946844 |
| Entity Address, Address Line One | 142 Sansome Street |
| Entity Address, Address Line Two | Second Floor |
| Entity Address, City or Town | San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94104 |
| City Area Code | (415) |
| Local Phone Number | 432-9270 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.001 per share |
| Trading Symbol | ETNB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
