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 …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. According to the United States Census Bureau, the city has a total area of 8.602 square miles, of which 8.522 square miles is land and 0.08 square miles is water. The Stillwater River runs on its east side.
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