There are 1 US Post Offices in Ziebach County serving a total of 1,042 residents equating to 1,042 residents per post office. It's estimated that approximately 1,663 packages are handled across the 1 county post offices annually. Ziebach County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 2,801. Its county seat is Dupree. It is the last county in the United States alphabetically. The county's per-capita income makes it the fourth poorest county in the United States. However, another study states that the county is the poorest in the nation."