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 (33.244534, -81.363214). 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.
U.S. Route 278 passes through the city, leading south 17 miles (27 km) to Allendale and northwest 42 miles (68 km) to Augusta, Georgia. State highways 3, 70, 64 also pass through the city; SC 64 leads west 6 miles (10 km) to the east entrance of the Savannah River Site, which is a nuclear reservation.
According to the United States Census Bureau, Barnwell has a total area of 8.0 square miles (20.7 km2), of which 7.8 square miles (20.3 km2) is land and 0.15 square miles (0.4 km2), or 1.86%, is water.
As of the census of 2000, there were 5,035 people, 2,035 households, and 1,353 families residing in the city. The population density was 659.5 people per square mile (254.8/km2). There were 2,304 housing units at an average density of 301.8 per square mile (116.6/km2). The racial makeup of the city was 49.81% White, 47.37% Black, 1.05% Asian, 0.40% Native American, 0.04% Pacific Islander, 0.34% from other races, and 0.99% from two or more races. Hispanic or Latino of any race were 0.62% of the population.