Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Union County is a county located in the U.S. state of South Dakota. It is part of the Sioux City, IA NE SD Metropolitan Statistical Area. As of the 2000 census, the population was 12,584. Its county seat is Elk Point. Originally named Cole County, the named was changed to Union because of Civil War sentiment.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."