| Document and Entity Information | May 14, 2024 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | May 14, 2024 |
| Entity Registrant Name | Mercer Bancorp, Inc. |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 000-56575 |
| Entity Tax Identification Number | 92-3452469 |
| Entity Address, Address Line One | 1100 Irmscher Blvd |
| Entity Address, City or Town | Celina |
| Entity Address State Or Province | OH |
| Entity Address, Postal Zip Code | 45822 |
| City Area Code | 419 |
| Local Phone Number | 586-5158 |
| 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 | 0001967306 |
| Amendment Flag | false |
