STOCK-BASED COMPENSATION - Share-based compensation expense (Details) - USD ($)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total share-based compensation expense		$ 0
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total share-based compensation expense	$ 668,000
Stock option expense | Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total share-based compensation expense	399,000
Restricted stock expense | Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total share-based compensation expense	$ 269,000