Dayton is a city in the U.S. state of Ohio and the county seat of Montgomery County, the fourth most populous county in the state. The population was 166,179 at the 2000 census. The Dayton Metropolitan Statistical Area had a population of 848,153 in the 2000 census.Dayton is the fourth largest metropolitan area in Ohio and the 61st largest Metropolitan Area in the United States. The Dayton-Springfield-Greenville Combined Statistical Area had a population of 1,085,094 in 2000. Dayton is situated within the Miami Valley region of Ohio, just north of the Cincinnati metropolitan area.