Aberdeen is a city in and the county seat of Brown County, South Dakota, United States, located approximately 125 miles northeast of Pierre. T…Aberdeen is a city in and the county seat of Brown County, South Dakota, United States, located approximately 125 miles northeast of Pierre. The population was 28,495 at the 2020 census, making it the third-most populous city in the state, after Sioux Falls and Rapid City. Aberdeen is home of Northern State University.