Charleston County is located in the U.S. state of South Carolina along the Atlantic coast. As of the 2020 census, the population was 408,235, making it the third-most populous county in South Carolina. Its county seat is Charleston. It is also the largest county in the state by total area, although Horry County has a larger land area. The county was created in 1800 by an act of the South Carolina State Legislature.

Charleston is the most populous city in the U.S. state of South Carolina, the county seat of Charleston County, and the principal city in the Charleston metropolitan area. The city lies just south of the geographical midpoint of South Carolina's coastline on Charleston Harbor, an inlet of the Atlantic Ocean formed by the confluence of the Ashley, Cooper, and Wando rivers. Charleston had a population of 150,227 at the 2020 census. The population of the Charleston metropolitan area, comprising Berkeley, Charleston, and Dorchester counties, was estimated to be 849,417 in 2023. It ranks as the third-most populous metropolitan statistical area in the state, and the 71st-most populous in the United States.