| Document and Entity Information | Feb. 18, 2025 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 18, 2025 |
| Entity Registrant Name | SEPTERNA, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-42382 |
| Entity Tax Identification Number | 84-3891440 |
| Entity Address Address Line 1 | Septerna, Inc. |
| Entity Address Address Line 2 | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.001 par value per share |
| Trading Symbol | SEPN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001984086 |
