STOCK-BASED COMPENSATION - Schedule of share-based compensation expense (Details) - Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Total share-based compensation expense	$ 2,879	$ 664
Stock option expense
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Total share-based compensation expense	1,598	422
Restricted stock expense
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Total share-based compensation expense	$ 1,281	$ 242