| Cover | Oct. 08, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 8, 2021 |
| Entity Registrant Name | BLUE FOUNDRY BANCORP |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-40619 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.01 par value |
| Trading Symbol | BLFY |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001846017 |
| Amendment Flag | false |
