Ennis is a town in Madison County, Montana, United States, in the southwestern part of the state. The population was 840 at the 2000 census. The elevation of Ennis is 4,941 feet (1,506 m) above sea level. U.S. Route 287 runs through town, following the Madison River as it descends from West Yellowstone. Just downstream from town is Ennis Lake, formed by the Madison Dam.
Prior to the arrival of Europeans, the Shoshone, Flathead, and Bannock tribes hunted the Madison Valley each spring. The Lewis and Clark Expedition viewed the Madison Valley in July 1805 from Three Forks, prior to ascending the Jefferson. It can be assumed that some of their party came through Ennis on their journey.
In 1863, gold was discovered in Alder Gulch. This brought on "the rush." Two months later, William Ennis homesteaded the site along the Madison River that was soon to become the town of Ennis, his namesake.