Hagerstown is a city in Washington County, Maryland. It is the county seat of Washington County, and by many definitions, the largest city in a region known as Western Maryland (if you exclude Frederick County from Western Maryland). The population of Hagerstown city proper at the 2010 census was 39,662, and the population of the Hagerstown-Martinsburg Metropolitan Area (extending into West Virginia) was 269,140. Hagerstown ranks as Maryland's sixth largest city.