Washington County is a county located in the southeastern part of the U.S. state of Ohio. As of the 2020 census, the population was 59,711. Its county seat is Marietta. The county, the oldest in the state, is named for George Washington. Washington County comprises the Marietta, OH Micropolitan Statistical Area, which is also included in the Parkersburg-Marietta-Vienna, WV-OH Combined Statistical Area.