Greenville is a city in Darke County, Ohio, United States, and its county seat. It is near Ohio's western edge, about 33 miles northwest of Day…Greenville is a city in Darke County, Ohio, United States, and its county seat. It is near Ohio's western edge, about 33 miles northwest of Dayton. The population was 12,786 at the 2020 census. Indigenous tribes in the region included the Wyandot, the Delaware, the Shawnee, the Ottawa, the Chippewa, the Pottawatomi, the Miami, the Wea, the Kickapoo, the Piankasha, the Kaskaskia and the Eel River. These participated in the Northwest Indian War, their effort to repel European Americans from the Northwest Territory.