BENEFIT PLANS - Share based compensation narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Aug. 25, 2022	Dec. 31, 2022	Dec. 31, 2021
Employee Stock Ownership Plan (ESOP) Disclosures [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		$ 21,200
Purchase of common shares by the ESOP		615	$ 1,000
ESOP and stock-based compensation expense		1,200	$ 1,300
Expected future expense of non-vested options		$ 10,300
Restricted Stock
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Expected period of recognition (in years)		4 years 8 months 12 days
Expected future expense of non-vested RSUs		$ 3,200
Stock option
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Expected period of recognition (in years)		6 years 3 months 18 days
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan)
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Number of shares authorized for grant (in shares)	3,993,150
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock
Employee Stock Ownership Plan (ESOP) Disclosures [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
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Vesting service period (in years)	5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Maximum
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Vesting service period (in years)	7 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Stock option
Employee Stock Ownership Plan (ESOP) Disclosures [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 | Minimum
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Vesting service period (in years)	5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Stock option | Maximum
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Vesting service period (in years)	7 years