Roberts County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 10,280. 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. It was created on March 8, 1883, and fully organized by August 6 of that year. Its boundary was altered once, in 1885.