v3.26.1
Document and Entity Information
12 Months Ended
Dec. 31, 2025
Document Information [Line Items]  
Document Type 11-K
Document Period End Date Dec. 31, 2025
Entity Registrant Name NB Bancorp, Inc.
Entity Central Index Key 0001979330
Document Fiscal Year Focus 2025
Amendment Flag false
Needham Bank 401(k) Plan  
Document Information [Line Items]  
Document Type 11-K
Document Period End Date Dec. 31, 2025
Document Fiscal Year Focus 2025
Amendment Flag false