MariettaMarietta is a city located in central Cobb County, Georgia, and is its county seat.As of the 2000 census, the city had a total population of 58,748, making it one of metro Atlanta's largest suburbs. Census estimates of 2008 indicate a population of 67,021. Marietta is the third-largest of three principal cities (by population) of and is included in the Atlanta–Sandy Springs–Marietta, Georgia, metropolitan statistical area, which is included in the Atlanta-Sandy Springs-Gainesville, Georgia-Alabama (part) combined statistical area.