STOCK-BASED COMPENSATION - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Aug. 25, 2022	Dec. 31, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Unallocated common stock held by ESOP (in shares)		2,281,800	2,281,800
Price per share, ESOP (in usd per share)		$ 10
Remaining principal on ESOP loan		$ 20,600	$ 21,200
Purchase of common shares by the ESOP		635	615
ESOP and stock-based compensation expense		895	$ 1,200
Expected future expense of non-vested options		$ 8,200
Stock options granted (in shares)		0
Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)		5 years 1 month 6 days
Expected future expense of non-vested RSUs		$ 6,200
Stock option expense
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)		5 years 3 months 18 days
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	3,993,150
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	1,140,900
Initial period before vesting begins (in years)	1 year
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	7 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Stock option expense
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	2,852,250
Initial period before vesting begins (in years)	1 year
Expiration period of options(in years)	10 years
Expected life of stock options (in years)		6 years 10 months 24 days
Risk free interest rate		3.94%
Expected volatility rate		29.41%
Expected dividend yield		0.88%
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Stock option expense | Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Stock option expense | Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	7 years