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Document and Entity Information - shares
 3 Months Ended
Mar. 31, 2024
May 06, 2024
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2024  
Document Fiscal Year Focus 2024  
Document Fiscal Period Focus Q1  
Entity Registrant Name 89bio, Inc.  
Entity Central Index Key 0001785173  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   98,383,998
Entity Shell Company false  
Entity Small Business false  
Entity Emerging Growth Company false  
Title of 12(b) Security Common stock, par value $0.001 per share  
Trading Symbol ETNB  
Security Exchange Name NASDAQ  
Entity File Number 001-39122  
Entity Incorporation, State or Country Code DE  
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  
Document Quarterly Report true  
Document Transition Report false