| Document and Entity Information | Jan. 04, 2023 |
| Cover [Abstract] | |
| Amendment Flag | true |
| Entity Central Index Key | 0001785173 |
| Document Type | 8-K/A |
| Document Period End Date | Jan. 04, 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 |
| Amendment Description | On January 6, 2023, 89bio, Inc. filed a Current Report on Form 8-K (the “Original 8-K”) to report, among other things, that it had entered into the Loan Agreement and agreed to issue the Warrant to K2HV on the 16th VWAP Trading Day following the Closing Date. This Current Report on Form 8-K/A amends the Original 8-K solely to (i) provide the conversion price for the Term Loan, (ii) provide the number of shares subject to the Warrant and the warrant price and (ii) file the form of Warrant as an exhibit. There are no other modifications or updates to any of the information made in the Original 8-K. All defined terms used in this Explanatory Note shall have the meanings set forth below. |
