Sullivan's Island, historically known as O'Sullivan's Island, is a town and island in Charleston County, South Carolina, United States, at the entrance to Charleston Harbor, with a population of 1,791 at the 2010 census, and 1,891 people in 2020. The town is part of the Charleston metropolitan area, and is considered a very affluent suburb of Charleston.

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.