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.

Lincoln County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 65,161, making it the third most populous county in South Dakota. Its county seat is Canton. The county was named for Abraham Lincoln, 16th President of the United States.