Marietta is a city in Washington County, Ohio, United States, and its county seat. It is located in southeastern Ohio at the confluence of the Muskingum and Ohio Rivers, 11 miles (18 km) northeast of Parkersburg, West Virginia. As of the 2020 census, Marietta has a population of 13,385 people. It is the principal city of the Marietta micropolitan area, which includes all of Washington County, and is the second-largest city in the Parkersburg–Marietta–Vienna combined statistical area.