There are 5 US Post Offices in Tripp County serving a total of 5,672 residents equating to 1,134 residents per post office. It's estimated that approximately 9,053 packages are handled across the 5 county post offices annually. Tripp County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 5,644. Its county seat is Winner. The county was created in 1873 and organized in 1909. It is named after lawyer, judge, and diplomat Bartlett Tripp."