Clay County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 14,967, and was estimated to be 15,245 in 2024. The county seat is Vermillion, which is also home to the University of South Dakota. The county is named for Henry Clay, American statesman, US Senator from Kentucky, and United States Secretary of State in the 19th century. Clay County comprises the Vermillion, SD Micropolitan Statistical Area, which is also included in the Sioux City-Vermillion, Iowa-SD-Nebraska Combined Statistical Area.

History

The future Clay County area was opened for legal settlement in 1859. In Autumn 1859, Ahira A. Partridge (who would become the first elected sheriff of the county) crossed the Missouri river into the Dakota territory, and became the first white man to settle, on 160 acres of land that now underlies Vermillion. In 1862 the county was formally organized. The Clay County Courthouse was built in 1912.

Clay County is the name of 17 other counties in the United States, most of them named for Henry Clay.

Geography

Clay County lies on the south line of South Dakota. The south boundary line of Clay County abuts the north line of the state of Nebraska (across the Missouri River). The Missouri River flows southeast along the south boundary line of Clay County. A small drainage creek flows into the county from Turner County, draining the central and eastern portions of the county and discharging into the river. Smaller drainages move water from the western county areas into the river. In addition to sloping into the drainage through the center of the county, the terrain generally slopes to the south. The area is largely devoted to agriculture.

According to the United States Census Bureau, the county has a total area of , of which is land and (1.3%) is water. It is the 66th largest county in South Dakota by total area. It is the smallest county by area in South Dakota.

Adjacent counties

  • Turner County – north
  • Lincoln County – northeast
  • Union County – east
  • Dixon County, Nebraska – southeast
  • Cedar County, Nebraska – southwest
  • Yankton County – west

Major highways

  • 20px South Dakota Highway 19
  • 20px South Dakota Highway 46
  • 20px South Dakota Highway 50

Protected areas

  • Missouri National Recreational River (part)
  • Spirit Mound State Historic Prairie

Demographics