Darke County is a county in the U.S. state of Ohio. As of the 2020 census, the population was 51,881. Its county seat and largest city is Greenville. The county was created in 1809 and later organized in 1817. It is named for William Darke, an officer in the American Revolutionary War. Darke County comprises the Greenville, OH Micropolitan Statistical Area, which is also included in the Dayton-Springfield-Sidney, OH Combined Statistical Area.