There are 8 US Post Offices in Roberts County serving a total of 9,698 residents equating to 1,212 residents per post office. It's estimated that approximately 15,479 packages are handled across the 8 county post offices annually. Roberts County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 10,149. Its county seat is Sisseton. The county was named either for S. G. Roberts of Fargo, North Dakota, or for Solomon Robar, an early local French fur trader."