Union is a city in Montgomery and Miami Counties in the U.S. state of Ohio. The population was 6,419 at the 2010 census. It is part of the Dayton Metropolitan Statistical Area.
Union is located at 39°54′1″N 84°18′49″W / 39.90028°N 84.31361°W / 39.90028; -84.31361 (39.900154, -84.313494).
According to the United States Census Bureau, the city has a total area of 4.3 square miles (11.1 km²).The Stillwater River runs on its east side.