Marysville is a city in and the county seat of Union County, Ohio, United States, approximately 27 miles (43 km) northwest of Columbus. The population was 25,571 at the 2020 census. It is part of the Columbus metropolitan area.

Union County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 62,784. Its county seat is Marysville. Its name is reflective of its origins, it being the union of portions of Franklin, Delaware, Madison, and Logan counties.