Washington County is a county located in the U.S. state of Maryland. The population was 154,705 as of the 2020 census. Its county seat and largest city is Hagerstown. The county is part of the Western Maryland region of the state. Washington County was the first county in the United States to be named for the Revolutionary War general George Washington. Washington County is one of three Maryland counties recognized by the Appalachian Regional Commission as being part of Appalachia.