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Cover Page - shares
 6 Months Ended
Jun. 30, 2025
Aug. 04, 2025
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2025  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Trading Symbol FSEA  
Title of 12(b) Security Common stock, $0.01 par value per share  
Security Exchange Name NASDAQ  
Entity Registrant Name First Seacoast Bancorp, Inc.  
Entity Central Index Key 0001943802  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   4,702,513
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Shell Company false  
Document Quarterly Report true  
Document Transition Report false  
Entity Incorporation, State or Country Code MD  
Entity File Number 001-41597  
Entity Address, Address Line One 633 Central Avenue  
Entity Address, City or Town Dover  
Entity Address, State or Province NH  
City Area Code 603  
Local Phone Number 742-4680  
Entity Address, Postal Zip Code 03820  
Entity Tax Identification Number 92-0334805