| Document and Entity Information | Nov. 14, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 14, 2022 |
| Entity Registrant Name | FIRST SEACOAST BANCORP |
| Entity Incorporation, State or Country Code | X1 |
| Entity File Number | 001-38985 |
| Entity Tax Identification Number | 84-2404519 |
| Entity Address, Address Line One | 633 Central Avenue |
| Entity Address, City or Town | Dover |
| Entity Address, State or Province | NH |
| Entity Address, Postal Zip Code | 03820 |
| City Area Code | 603 |
| Local Phone Number | 742-4680 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | FSEA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001769267 |
