Pierre is the capital city of the U.S. state of South Dakota and the county seat of Hughes County. The population was 14,091 at the 2020 cens…Pierre is the capital city of the U.S. state of South Dakota and the county seat of Hughes County. The population was 14,091 at the 2020 census, making it the 2nd least populous US state capital after Montpelier, Vermont. It is South Dakota's 9th most populous city. Pierre is the principal city of the Pierre Micropolitan Statistical Area, which includes all of Hughes and Stanley counties.