Of 66 counties in the Mount Rushmore State, Union County ranks 62nd by size (467 square miles). Union County is in Southeast South Dakota. South Dakota's Union County has about 1,000 acres of lakefront homes and property for sale based on recent Land Network data. The combined market value of land and rural real estate for sale here was about $2 million. Browse all waterfront property for sale in Union County on the Land Network to find the marina, lakefront and other recreational property that meets your needs, from waterside land perfect as a retirement investment to existing residential waterfront land and property for hunting, fishing and other outdoor-recreation pursuits! The most common type of land for sale by combined acreage in Union County is residential property.