| Cover | Oct. 08, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 8, 2021 |
| Entity File Number | 001-40893 |
| Entity Registrant Name | Catalyst Bancorp, Inc. |
| Entity Central Index Key | 0001849867 |
| Entity Tax Identification Number | 86-2411762 |
| Entity Incorporation, State or Country Code | LA |
| Entity Address, Address Line One | 235 N. Court Street |
| Entity Address, City or Town | Opelousas |
| Entity Address, State or Province | LA |
| Entity Address, Postal Zip Code | 70570 |
| City Area Code | 337 |
| Local Phone Number | 948-3033 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | CLST |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | false |
