Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2022	Nov. 07, 2022
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q3
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	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		46,536,970
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	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