Johns Island is an island in Charleston County, South Carolina, United States, and is the largest island in the state of South Carolina. Johns Island is bordered by the Wadmalaw, Seabrook, Kiawah, Edisto, Folly, and James islands; the Stono and Kiawah rivers separate Johns Island from its border islands. It is the fourth-largest island on the US east coast, surpassed only by Long Island, Mount Desert Island and Martha's Vineyard. Johns Island is 84 square miles (220 km2) in area, with a population of 21,500.

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.