Cover - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Mar. 07, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-40619
Entity Registrant Name	BLUE FOUNDRY BANCORP
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	86-2831373
Entity Address, Address Line One	19 Park Avenue,
Entity Address, City or Town	Rutherford,
Entity Address, State or Province	NJ
Entity Address, Postal Zip Code	07070
City Area Code	201
Local Phone Number	939-5000
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	BLFY
Security Exchange Name	NASDAQ
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 Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		28,522,500
Documents Incorporated by Reference	None
Entity Central Index Key	0001846017
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Public Float			$ 0