There are 40 US Post Offices in Rockingham County serving a total of 295,223 residents equating to 7,381 residents per post office. It's estimated that approximately 471,209 packages are handled across the 40 county post offices annually. Rockingham County is a county located in the U.S. state of New Hampshire. As of the 2010 census, the population was 295,223, making it the second-most populous county in New Hampshire. The county seat is Brentwood. Rockingham County is included in the Boston-Cambridge-Newton, MA-NH Metropolitan Statistical Area.