There are 2 US Post Offices in Sully County serving a total of 1,148 residents equating to 574 residents per post office. It's estimated that approximately 1,832 packages are handled across the 2 county post offices annually. Sully County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 1,373, making it the third-least populous county in South Dakota. Its county seat is Onida. The county was created in 1873 and organized in 1883. It is named after General Alfred Sully who built Fort Sully. Sully County is included in the Pierre, SD Micropolitan Statistical Area."