There are 7 US Post Offices in Yankton County serving a total of 3,728 residents equating to 533 residents per post office. It's estimated that approximately 5,950 packages are handled across the 7 county post offices annually. Yankton County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 22,438. Its county seat is Yankton. Yankton County comprises the Yankton, SD Micropolitan Statistical Area."