| Document and Entity Information | Nov. 07, 2024 |
| Document And Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 07, 2024 |
| 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 1 | 655 Montgomery Street |
| Entity Address Address Line 2 | Suite 1500 |
| Entity Address City Or Town | San Francisco |
| Entity Address State Or Province | CA |
| Entity Address Postal Zip Code | 94111 |
| 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 | false |
| Entity Central Index Key | 0001785173 |
| Amendment Flag | false |
| Former Address [Member] | |
| Document And Entity Information [Line Items] | |
| Entity Address Address Line 1 | 142 Sansome Street |
| Entity Address Address Line 2 | Second Floor |
| Entity Address City Or Town | San Francisco |
| Entity Address State Or Province | CA |
| Entity Address Postal Zip Code | 94104 |
