Pennington County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 109,222, making it the second-most populous county in South Dakota. Its county seat is Rapid City. The county was created in 1875, and was organized in 1877. It is named for John L. Pennington, fifth Governor of Dakota Territory, who held office in 1875 when the county was formed. Pennington County is included in the Rapid City metropolitan statistical area.