Barnwell is a city in Barnwell County, South Carolina, United States, located along U.S. Route 278. The population was 5,035 at the 2000 census. It is the county seat of Barnwell County.
Barnwell is located at 33°14′40″N 81°21′48″W / 33.24444°N 81.36333°W / 33.24444; -81.36333 (33.244534, -81.363214).
According to the United States Census Bureau, the city has a total area of 7.8 square miles (20.2 km²), of which, 19.8 square kilometers (7.64 sq mi) of it is land and 0.1 square miles (0.4 km²) of it (1.80%) is water.