Rapid City, South Dakota - Wikipedia Rapid City (Lakota: Mni Lúzahaŋ Otȟúŋwahe; "Swift Water City") is the second most populous city in South Dakota and the county seat of Pennington County. Named after Rapid Creek, where the settlement developed, it is in western South Dakota, on the Black Hills' eastern slope. The population was 74,703 as of the 2020 Census.. Known as the "Gateway to the Black Hills" and …