county, western South Carolina, U.S. It consists of a piedmont region bordered to the west by the Savannah River and its impoundment, J. Strom Thurmond Lake, which it shares with the state of Georgia. Most of the county lies within the southern section of Sumter National Forest, which includes swampy bottomlands along rivers as well as hilly, pine-topped areas. Within the forest are Hickory Knob, Baker Creek, and Hamilton Branch state parks. The Savannah River Scenic Highway runs through the western part of the county.

Yuchi Indians lived in the area in the 17th century; Indian raids by the Yuchi and others throughout the mid-18th century slowed European settlement. Gold mining and cotton growing were important in the 19th century, but in time the gold was exhausted, and—because of erosion, boll weevil infestations, and economic factors—the land reverted to forest in the 20th century. The county was organized in 1916 and named for Cyrus Hall McCormick (McCormick, Cyrus Hall), inventor of the mechanical reaper, who donated to the county seat, McCormick, much of the land for the town.

The county's economy depends on lumbering and the textile industry. McCormick county has the smallest area and the least population of any county in the state. Area 359 square miles (931 square km). Pop. (2000) 9,958; (2007 est.) 10,098.