Clark County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 3,837. Its county seat is Clark. The county was created in 1873 and organized in 1881. It was named for Newton Clark, a Dakota Territory legislator in 1873.

South Dakota Highway 20 (SD 20) is a 385.079-mile (619.725 km) state highway in northern South Dakota, United States, that connects the Montana state line, west-southwest of Camp Crook, with the Minnesota state line, east-southeast of Revillo, via Buffalo, Bison, Timber Lake, Mobridge, Selby, and Watertown. From Mobridge to the Walworth–Potter county line, this highway is part of the Lewis and Clark Trail. From about Timber Lake to just west of Mobridge, this highway is part of the Native American Scenic Byway. From the Montana state line to Camp Crook, the highway is a dirt road.