Fort Pierre is a city in Stanley County, South Dakota, United States. It is part of the Pierre, South Dakota Micropolitan Statistical Area. The population was 1,991 at the 2000 census. It is the county seat of Stanley County. Lily Park, in Fort Pierre, is situated at the mouth of the Bad River. The site where the Lewis and Clark Expedition met with the Teton Sioux is located nearby. It derives its name from Pierre Chouteau, Jr. , a fur trader who established the fort.