Barnwell is a city in and county seat of Barnwell County, South Carolina, United States, located along U.S. Route 278. The population was 4,7…Barnwell is a city in and county seat of Barnwell County, South Carolina, United States, located along U.S. Route 278. The population was 4,750 at the 2010 census. Barnwell is located east of the center of Barnwell County at 33°14′40″N 81°21′48″W / 33.24444°N 81.36333°W. Turkey Creek, a tributary of the Salkehatchie River, runs through the city just west of the downtown, and includes a small impoundment known as Lake Brown in the north part of the city.