Fort Pierre is a city in Stanley County, South Dakota, United States. It is part of the Pierre, South Dakota Micropolitan Statistical Area and the county seat of Stanley County. The population was 2,078 at the 2010 census. Lily Park in Fort Pierre is situated at the confluence of the Bad and Missouri rivers. The city was named for Pierre Chouteau, Jr. , a fur trader from St. Louis who established the trading post and fort.