- Dayton (/ˈdeɪtən/) is a city in and the county seat of Montgomery County, Ohio, United States. A small part of the city extends into Greene County. As of the 2020 census, the city proper had a population of 137,644, making it the sixth-most populous city in Ohio. Dayton anchors the state’s fourth-largest metropolitan area, the Dayton metropolitan area, which had 814,049 residents. Dayton is located within Ohio’s Miami Valley region, 50 miles (80 km) north of Cincinnati and 60 miles (97 km) west of Columbus. It is a principal city of the Dayton–Springfield–Sidney combined statistical area, home to a population of 1,086,512.