Clay County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 14,967. 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.

Union County is the southernmost county in the U.S. state of South Dakota. As of the 2020 census, the population was 16,811, making it the 13th most populous county in South Dakota. Its county seat has been Elk Point since April 30, 1865. Originally named Cole County, its name was changed to Union on January 7, 1864, because of Civil War sentiment.