The Selway-Bitterroot Wilderness is the third largest wilderness in the lower 48 states, surpassed in size only by California's Death Valley Wilderness and Idaho's Frank Church-River of No Return Wilderness (RONR). Less than one-fifth of the area lies in Montana (see Idaho, Selway-Bitterroot Wilderness).
Only the 600-foot-wide Nez Perce Trail (The Magruder Corridor), an unimproved dirt road, separates the Selway-Bitterroot from the Frank Church-RONR. This Wilderness straddles both sides of the Bitterroot Mountain Range, which stands along the Montana-Idaho border and includes the Wild and Scenic Selway River, all of which flows through Idaho.
A small portion of this wilderness is within the Lolo National Forest. Montana access is mostly through the Bitterroot National Forest.