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.

Potter County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 2,472. Its county seat is Gettysburg. The county was created in 1873 and organized in 1883.