Greenville County is a county located in the state of South Carolina, in the United States. As of the 2010 census, the population was 451,225, making it the most populous county in the state. In 2017, the estimated population of the county was 506,837. Its county seat is Greenville. The county is also home to the Greenville County School District, the largest school system in South Carolina. County government is headquartered at Greenville County Square.