The Pioneer Mountains are a mountain range in the U.S. state of Idaho, spanning Blaine, Butte and Custer counties. The range is bounded on the west by the Big Wood River, Trail Creek and Summit Creek, and the North Fork Big Lost River, on the north and east by the East Fork Big Lost River, Left Fork Cherry Creek, Cherry Creek, Dry Fork Creek, Saint Louis Canyon, and Champagne Creek, and on the south by the Snake River Plain.