Barnwell County is a county in the U.S. state of South Carolina. As of the 2020 census, its population was 20,589. It is located in the Central Savannah River Area. Its county seat is Barnwell.

South Carolina is a state in the coastal Southeastern region of the United States. It borders North Carolina to the north, the Atlantic Ocean to the southeast, and Georgia to the southwest across the Savannah River. Along with North Carolina, it makes up the Carolinas region of the East Coast. South Carolina is the 40th-largest and 23rd-most populous U.S. state with a recorded population of 5,118,425 according to the 2020 census. In 2019, its GDP was $213.45 billion. South Carolina is composed of 46 counties. The capital is Columbia with a population of 136,632 in 2020; while its most populous city is Charleston with a 2020 population of 150,227. The Greenville-Spartanburg-Anderson, SC Combined Statistical Area is the most populous combined metropolitan area in the state, with an estimated 2023 population of 1,590,636.