Located in the heart of central New Jersey, Franklin Township covers nearly 47 square miles. It is bounded to the north and east by the Raritan River, and to the west and south by the Millstone River. This extensive river frontage has historically defined the township's borders, economy, and ecology.
The township sits in a transition zone between the Piedmont Plateau and the Inner Coastal Plain. As a result, the landscape features a mix of broad river valleys, fertile agricultural plains, and gentle woodlands. Notably, Franklin Township contains large tracts of preserved natural resources, including Six Mile Run Reservoir Site—a massive state park that was originally slated to be a reservoir but now serves as an extensive public trail network and protected wildlife habitat.