Pierre is the capital city of the U.S. state of South Dakota and the seat of Hughes County. As of the 2020 census, its population was 14,091. Pi…Pierre is the capital city of the U.S. state of South Dakota and the seat of Hughes County. As of the 2020 census, its population was 14,091. Pierre is the ninth-most populous city of South Dakota, and the second-least populous U.S. state capital.