Rapid City is a city in and the county seat of Pennington County, South Dakota, United States. It is located on the eastern slope of the Black Hills in western South Dakota and was named after Rapid Creek, where the settlement developed. It is the second-most populous city in the state (after Sioux Falls) with a population of 74,703 as of the 2020 census. The Rapid City metropolitan area has 156,000 residents.