Montgomery County is in the southwestern part of the U.S. state of Ohio. At the 2020 census, the population was 537,309, making it the fifth-most populous county in Ohio. The county seat is Dayton. The county was named in honor of Richard Montgomery, an American Revolutionary War general, who was killed in 1775 while attempting to capture Quebec City, Canada. Montgomery County is part of the Dayton, Ohio, Metropolitan Statistical Area.

Dayton 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. It 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.