v3.25.2
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2025
Jul. 31, 2025
Document And Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2025  
Document Transition Report false  
Entity File Number 001-37852  
Entity Registrant Name PROTAGONIST THERAPEUTICS, INC.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 98-0505495  
Entity Address, Address Line One 7707 Gateway Boulevard, Suite 140  
Entity Address, City or Town Newark  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94560-1160  
City Area Code 510  
Local Phone Number 474-0170  
Title of 12(b) Security Common Stock, par value $0.00001  
Trading Symbol PTGX  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   62,211,318
Entity Central Index Key 0001377121  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Amendment Flag false