Rockingham County is a county in the U.S. state of New Hampshire. As of the 2020 census, the population was 314,176, making it New Hampshire's second-most populous county. The county seat is Brentwood. Rockingham County is part of the Boston-Cambridge-Newton, MA-NH Metropolitan Statistical Area and the greater Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area. Per the 2020 census, it was New Hampshire's fastest growing county from 2010 to 2020.