New Castle is a town in Rockingham County, New Hampshire, United States. The population was 968 at the 2010 census. It is the smallest and easternmost town in New Hampshire, and the only one located entirely on islands. It is home to Fort Constitution Historic Site, Fort Stark Historic Site, and the New Castle Common, a 31-acre recreation area on the Atlantic Ocean. New Castle is also home to a United States Coast Guard station, as well as the historic Wentworth by the Sea hotel.