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.

Derry is a town in Rockingham County, New Hampshire, United States. The population was 34,317 at the 2020 census. Although it is a town and not a city, Derry is the most populous community in Rockingham County and the 4th most populous in the state. The town's nickname, "Spacetown", derives from the fact that Derry is the birthplace of Alan Shepard, the first astronaut from the United States in space. Derry was also for a time the home of the poet Robert Frost and his family.