Corson County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 3,902. Its county seat is McIntosh. The county was named for Dighton Corson, a native of Maine, who came to the Black Hills in 1876, and in 1877 began practicing law at Deadwood. The county is encompassed within the Standing Rock Indian Reservation, which extends into North Dakota.