U.S. Highway 12 (US 12) is an east–west United States Numbered Highway in the state of Montana. It extends approximately 597.6 miles (961.7 km) from the Idaho state line east to the North Dakota state line, the greatest distance that US 12 traverses through any state. US 12 in Montana has been defined as the Lewis and Clark Highway, despite not being the route followed by Lewis and Clark across the state.