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Cover
Oct. 20, 2025
Cover [Abstract]  
Document Type 8-K
Amendment Flag false
Document Period End Date Oct. 20, 2025
Entity File Number 001-41899
Entity Registrant Name NB BANCORP, INC.
Entity Central Index Key 0001979330
Entity Tax Identification Number 93-2560883
Entity Incorporation, State or Country Code MD
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
Elected Not To Use the Extended Transition Period false