There are 5 US Post Offices in Union County serving a total of 15,279 residents equating to 3,056 residents per post office. It's estimated that approximately 24,387 packages are handled across the 5 county post offices annually. Union County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 14,399. Its county seat is Elk Point. Originally named Cole County, the named was changed to Union because of Civil War sentiment. Union County is part of the Sioux City, IA\u2013NE\u2013SD Metropolitan Statistical Area. Progressive Farmer rated Union County second in the 2006 \"Best Place to Live\" in the U.S., because \"its schools are good, its towns neat and its people friendly.\""