county, southern South Carolina, U.S. It consists of a low-lying region on the Coastal Plain bordered to the northeast by the South Fork Edisto River and to the southwest by the Savannah River border with Georgia. The county is also drained by the Salkehatchie River. Wetlands and pine woods dominate the region. The largest portion of the U.S. Department of Energy's Savannah River Site, once a major producer of nuclear weapons materials and now operating in a much-diminished capacity, is in western Barnwell county. Barnwell State Park lies within the county's borders.

When the county was established in 1798, it was a region of plantations named for John Barnwell, who early in the 18th century had led settlers in subduing a Tuscarora Indian uprising. In 1865, during the American Civil War, Federal troops occupied and set fire to the county seat, the town of Barnwell.

An agricultural region, Barnwell county produces peanuts (groundnuts), watermelons, vegetables, cotton, grains, and livestock. The manufacture of textile products, refrigeration equipment, and rubber and plastic products is also important to the economy. Area 549 square miles (1,421 square km). Pop. (2000) 23,478; (2007 est.) 22,975.