Walworth County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 5,315. [1] Its county seat is Selby. [2] The county was created in 1873 and organized in 1883. [3] It is named for Walworth County, Wisconsin. [4] The Missouri River flows southward along the county's west boundary line. Swan Creek flows ...