Cover - shares	6 Months Ended
	Jun. 30, 2021	Jul. 30, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
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
Entity Current Reporting Status	No
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
Entity Central Index Key	0001846017
Amendment Flag	false
Document Fiscal Year Focus	2021
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q2