Union is a city in Montgomery and Miami Counties in the U.S. state of Ohio. The population was 6,859 at the time of the 2020 census. It is part of the Dayton Metropolitan Statistical Area.

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.