Marietta is a city in Washington County, Ohio, United States. The population was 14,515 at the 2000 census. It is the county seat of Washington County and 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. According to the United States Census Bureau, the city has a total area of 22.2 km² (8.6 mi²). 21.5 km² (8.3 mi²) of it is land and 0.7 km² (0.3 mi²) of it (3.03%) is water.

As of the census of 2000, there were 14,515 people, 5,904 households, and 3,501 families residing in the city. The population density was 674.4/km² (1,747.0/mi²). There were 6,609 housing units at an average density of 307.1/km² (795.4/mi²). The racial makeup of the city was 96.31% White, 1.08% African American, 0.46% Native American, 0.71% Asian, 0.06% Pacific Islander, 0.28% from other races, and 1.11% from two or more races. Hispanic or Latino of any race were 0.79% of the population.