Cover Page - shares	3 Months Ended
	Mar. 31, 2022	May 03, 2022
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q1
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
Entity Central Index Key	0001769267
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		6,065,999
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	X1
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	84-2404519
Entity File Number	001-38985