High Quality Content by WIKIPEDIA articles! Marietta is a city in and the county seat of Washington County, Ohio, United States. During 1788, pioneers to the Ohio Country established Marietta as the first permanent American settlement of the new United States in the Northwest Territory. Marietta is located in southeastern Ohio at the mouth of the Muskingum River at the confluence of the Ohio and Muskingum rivers. The population was 14,085 at the 2010 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.