v3.26.1
Document and Entity Information - shares
 3 Months Ended
Mar. 31, 2026
May 07, 2026
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Transition Report false  
Amendment Flag false  
Document Period End Date Mar. 31, 2026  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q1  
Current Fiscal Year End Date --12-31  
Entity File Number 001-42382  
Entity Registrant Name Septerna, Inc.  
Entity Central Index Key 0001984086  
Entity Tax Identification Number 84-3891440  
Title of 12(b) Security Common Stock, par value $0.001 per share  
Trading Symbol SEPN  
Security Exchange Name NASDAQ  
Entity Incorporation, State or Country Code DE  
Entity Address, Address Line One 250 East Grand Avenue  
Entity Address, City or Town South San Francisco  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94080  
City Area Code 650  
Local Phone Number 338-3533  
Entity Filer Category Non-accelerated Filer  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Shell Company false  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   45,075,533