Ada is a village in Hardin County, Ohio, United States, located about 69 miles (111 km) southwest of Toledo. The population was 5,334 at the 2020 census. It is the home of Ohio Northern University.

Hardin County is a county located in the west central portion of the U.S. state of Ohio. As of the 2020 census, the population was 30,696. Its county seat and largest city is Kenton. The county was created in 1820 and later organized in 1833. It is named for John Hardin, an officer in the American Revolution.