Charlestown is a town in Sullivan County, New Hampshire, United States. The population was 4,806 at the 2020 census, down from 5,114 at the 2010 census. The town is home to and the headquarters of the Student Conservation Association. The primary village in town, where 1,078 people resided at the 2020 census, is defined as the Charlestown census-designated place (CDP) and is located along New Hampshire Route 12. The town also includes the villages of North Charlestown, South Charlestown and Hemlock Center.