Marion County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 65,359. Its county seat is Marion. The county was created in 1820 and later organized in 1824. It is named for General Francis "The Swamp Fox" Marion, a South Carolinian officer in the Revolutionary War.

Marion is a city in and the county seat of Marion County, Ohio, United States. It is located in north-central Ohio, approximately 50 miles (80 km) north of Columbus. The population was 35,999 at the 2020 census, down slightly from 36,837 at the 2010 census. It is the largest city in Marion County and the principal city of the Marion micropolitan area. It is also part of the larger Columbus–Marion–Zanesville, OH Combined Statistical Area.