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.
The area that today is Rockingham County was first settled by Europeans moving north from the Plymouth Colony in Massachusetts as early as 1623. The government was linked tightly with Massachusetts until New Hampshire became a separate colony in 1679, but counties weren't introduced until 1769.
Rockingham was identified in 1769 as one of five original counties for the colony. It is named for Charles Watson-Wentworth, 2nd Marquess of Rockingham, who had been Prime Minister in 1765-1766. The county was organized in 1771, with its county seat at Exeter. In 1844 its area was reduced by the formation of Belknap County to the northwest. In 1997 the county court facilities were moved to Brentwood, a rural town adjacent to Exeter.