Sully County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 1,446, making it the fifth-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.

U.S. Highway 83, part of the United States Numbered Highway System, runs from the Mexican border in Brownsville, Texas, to the Canadian border near Westhope, North Dakota. In the state of South Dakota, it runs from Olsonville to Herreid by way of Mission, Murdo, and Gettysburg. The route also runs through the capital Pierre as well as its suburb Fort Pierre. For much of its length, US 83 runs through rural areas, passing through the badlands and small portions of the Black Hills of the state.