| Document and Entity Information | Jan. 22, 2025 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 22, 2025 |
| Entity Registrant Name | NB BANCORP, INC. |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 001-41899 |
| Entity Tax Identification Number | 93-2560883 |
| Entity Address, Address Line One | 1063 Great Plain Avenue |
| Entity Address, City or Town | Needham |
| Entity Address State Or Province | MA |
| Entity Address, Postal Zip Code | 02492 |
| City Area Code | 781 |
| Local Phone Number | 444-2100 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, Par Value $0.01 Per Share |
| Trading Symbol | NBBK |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001979330 |
| Amendment Flag | false |
