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.

Charles Watson-Wentworth, 2nd Marquess of Rockingham, was a British Whig statesman and magnate, most notable for his two terms as prime minister of Great Britain. He became the patron of many Whigs, known as the Rockingham Whigs, and served as a leading Whig grandee. He served in only two high offices during his lifetime but was nonetheless very influential during his one and a half years of service.