Dayton is a city in Montgomery County, Ohio, United States, and its county seat. It is the sixth-most populous city in Ohio, with a population of 137,644 at the time of 2020 United States census. The Dayton metropolitan area has an estimated 822,000 residents and is the state's fourth-largest metropolitan area. Dayton is located within Ohio's Miami Valley region, 40 miles (64 km) north of Cincinnati and 55 miles (89 km) southwest of Columbus, Ohio.