Frederick is a city in, and the county seat of, Frederick County, Maryland, United States. Frederick's population was 78,171 people as of the 20…Frederick is a city in, and the county seat of, Frederick County, Maryland, United States. Frederick's population was 78,171 people as of the 2020 census, making it the second-largest incorporated city in Maryland behind Baltimore. It is a part of the Washington metropolitan area and the greater Washington–Baltimore combined statistical area.