Stanley County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 2,980. Its county seat is Fort Pierre. The county was created in 1873, and was organized in 1890. It is named for David S. Stanley, a commander at Fort Sully from 1866 to 1874, which was located nearby. Stanley County is included in the Pierre, SD Micropolitan Statistical Area.