Rock Hill is the most populous city in York County, South Carolina, United States, and the 5th-most populous city in the state. It is also the 4th-…Rock Hill is the most populous city in York County, South Carolina, United States, and the 5th-most populous city in the state. It is also the 4th-most populous city of the Charlotte metropolitan area, behind Charlotte, Concord, and Gastonia. As of the 2020 census, the city's population was 74,372.