Fan Mountain is a prominent 10,312-foot peak in southwestern Montana's Madison Range, within Beaverhead National Forest. Near the range's northern end and east of Ennis Lake, its isolation and steep relief make it a clear landmark from Ennis and the Madison River valley. It also shapes the local watershed, with Jack Creek and Cedar Creek rising on opposite sides of the mountain as tributaries of the Madison River. The peak lies along the northern boundary of the Spanish Peaks parcel of the Lee Metcalf Wilderness. Geologically, Fan Mountain is a laccolith, formed when magma pushed into overlying rock without erupting, like nearby Lone Mountain.