X0508 4 2025-06-09 0001979330 NB Bancorp, Inc. NBBK 0001222378 CAMPANELLI JOSEPH P 1063 GREAT PLAIN AVE NEEDHAM MA 02492 1 1 1 President & CEO Chairman of the Board 0 Common Stock 2025-06-09 4 P 0 1000 16.03 A 330146 D Common Stock 2025-06-09 4 P 0 100 16.33 A 330246 D Common Stock 2025-06-09 4 P 0 900 16.35 A 331146 D Common Stock 2025-06-09 4 P 0 5000 16.29 A 45000 I By IRA Common Stock 1877 I By ESOP Common Stock 47788 I By 401(k) Includes shares of restricted stock which vest at a rate of 20% per year commencing on April 24, 2026. Reflects transactions not required to be reported pursuant to Section 16 of the Securities Act of 1934, as amended. /s/ Steven Lanter, pursuant to power of attorney 2025-06-10