Marietta is a city in the U.S. state of Ohio and the county seat of Washington County. The municipality is located in southeastern Ohio at the confluence of the Muskingum River with the Ohio River. The population was 14,515 at the 2000 census. It is the second largest by population of three principal cities of and included in the Parkersburg-Marietta-Vienna, West Virginia-Ohio (part) Metropolitan Statistical Area. Founded in 1788, Marietta is the oldest city in Ohio.