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 (excluding adjacent Frederick County). 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 incorporated city.