| Document and Entity Information | Oct. 27, 2023 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 27, 2023 |
| Entity Registrant Name | CFSB Bancorp, Inc. /MA/ |
| Entity Incorporation, State or Country Code | X1 |
| Securities Act File Number | 001-41220 |
| Entity Tax Identification Number | 87-4396534 |
| Entity Address, Address Line One | 15 Beach Street |
| Entity Address, City or Town | Quincy |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02170 |
| City Area Code | 617 |
| Local Phone Number | 471-0750 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001879103 |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | CFSB |
| Security Exchange Name | NASDAQ |
