Union Township is one of the eleven townships of Warren County, Ohio, United States, located in the central part of the county. It was established January 3, 1815 and named Union as it was formed from parts of Deerfield and Turtlecreek Townships. The 2000 census found 4,668 people in the township, up slightly from the 4,631 in 1990. Of those, 2,230 lived in the unincorporated part of the township and 2,438 lived in the village of South Lebanon.