 According to the United States Census Bureau, the township has a total area of 14.6 square miles (37.9 km²), of which, 14.6 square miles (37.7 km²) of it is land and 0.1 square miles (0.2 km²) of it (0.61%) is water.Union Township is bordered by Penn Township to the north.The Susquehanna River forms the eastern border of the township with Northumberland County.