Lawrence County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 25,768, making it the 8th most populous county in South Dakota. Its county seat is Deadwood. Lawrence County is coextensive with the Spearfish, SD Micropolitan Statistical Area, which is also included in the Rapid City-Spearfish, SD Combined Statistical Ar…Lawrence County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 25,768, making it the 8th most populous county in South Dakota. Its county seat is Deadwood. Lawrence County is coextensive with the Spearfish, SD Micropolitan Statistical Area, which is also included in the Rapid City-Spearfish, SD Combined Statistical Area.