New Castle is a town in Rockingham County, New Hampshire, United States. The population was 1,000 at the 2020 census. It is the easternmost town in New Hampshire and the smallest by area, and it is the only town in the state located entirely on islands. It is home to Fort Constitution Historic Site, Fort Stark Historic Site, and the New Castle Common, a 31-acre (13 ha) 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.