Yankton is a city in and the county seat of Yankton County, South Dakota, United States. The population was 15,411 at the 2020 census, making it the 7th most populous city in South Dakota, and it is the principal city of the Yankton Micropolitan Statistical Area, which includes the entirety of Yankton County and which had an estimated population of 23,297 as of July 1, 2021.