There are 2 US Post Offices in Clay County serving a total of 12,699 residents equating to 6,350 residents per post office. It's estimated that approximately 20,269 packages are handled across the 2 county post offices annually. Clay County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 13,864. The county seat is Vermillion, which is also home to the University of South Dakota. The county is named in honor of Henry Clay, American statesman, member of the United States Senate from Kentucky and United States Secretary of State in the 19th century. Clay County comprises the Vermillion, SD Micropolitan Statistical Area, which is also included in the Sioux City-Vermillion, IA-SD-NE Combined Statistical Area."