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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Apr. 21, 2026
Jun. 30, 2025
Cover [Abstract]      
Document Type 10-K/A    
Amendment Flag true    
Amendment Description On March 20, 2026, First Seacoast Bancorp, Inc. (the “Company”) filed its Annual Report on Form 10-K for the fiscal year ended December 31, 2025 (the “Original Form 10-K”). The purpose of this Amendment No. 1 on Form 10-K/A (the “Amended Form 10-K”) is to file the information required by Part III of Form 10-K. This Amended Form 10-K does not amend or restate any financial statements or other disclosures contained in the Original Form 10‑K. The Amended Form 10-K should be read together with the Original Form 10-K.    
Document Period End Date Dec. 31, 2025    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Entity Registrant Name First Seacoast Bancorp, Inc.    
Entity Central Index Key 0001943802    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Shell Company false    
Entity Emerging Growth Company false    
Entity Small Business true    
Trading Symbol FSEA    
Security Exchange Name NASDAQ    
Title of 12(b) Security Common stock, par value $0.01 per share    
Entity Common Stock, Shares Outstanding   4,691,674  
Entity Public Float     $ 48.8
Entity Incorporation, State or Country Code MD    
Entity File Number 001-41597    
Entity Tax Identification Number 92-0334805    
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    
Document Annual Report true    
Document Transition Report false    
Document Financial Statement Error Correction [Flag] false