Aberdeen is the county seat of Monroe County, Mississippi, United States. As of the 2020 census, the population was 4,961, down from 5,612 in 2010. Located on the banks of the Tombigbee River, Aberdeen was one of the busiest Mississippi ports of the 19th century. Cotton was heavily traded in town, and for a time Aberdeen was Mississippi's second largest city. Aberdeen retains many historic structures from this period, with over 200 buildings on the National Register of Historic Places.