Washington County is located in the western part of the U.S. state of Maryland. As of the 2020 census, the population was 154,705. It is the most populous county in the Western Maryland region and its county seat is Hagerstown.

Western Maryland, also known as the Maryland panhandle, is the portion of the U.S. state of Maryland that typically consists of Washington, Allegany, and Garrett counties. The region is bounded by Preston County, West Virginia, to the west, the Mason–Dixon line (Pennsylvania) to the north, and the Potomac River and West Virginia to the south. At one point, at the town of Hancock, the northern and southern boundaries are separated by just 1.8 miles, the narrowest stretch in the state.