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.