There are 3 US Post Offices in Jackson County serving a total of 2,248 residents equating to 749 residents per post office. It's estimated that approximately 3,588 packages are handled across the 3 county post offices annually. Jackson County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 3,031. Its county seat is Kadoka. The county was created in 1883 and organized in 1915. Washabaugh County was merged into Jackson County in 1983.