Defiance County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 38,286. Its county seat and largest city is Defiance. The county was named after an early Army fortification, Fort Defiance, which was so named by Mad Anthony Wayne to signify the settlers' "defiance" of the Indians. The Defiance, OH Micropolitan Statistical Area includes all of Defiance County.