Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 28, 2020	Jun. 30, 2019
Document And Entity Information
Document Type	10-K
Document Period End Date	Dec. 31, 2019
Entity Registrant Name	PROTAGONIST THERAPEUTICS, INC.
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 212.4
Entity Common Stock, Shares Outstanding		27,294,299
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Central Index Key	0001377121
Current Fiscal Year End Date	--12-31
Amendment Flag	false