The Bob Marshall Wilderness is located in western Montana in the United States and is named after Bob Marshall (1901-1939), an early forester, conservationist and co-founder of The Wilderness Society. The Bob Marshall Wilderness extends for 60 miles (95 km) along the Continental Divide and consists of 1,009,356 acres (4,085 km²). No roads cross the area. The wilderness was originally part of the South Fork, Pentagon and Sun River primitive areas which were combined into the wilderness in 1964. Along with the adjoining Scapegoat and Great Bear wildernesses, these three areas make up the Bob Marshall Wilderness Complex and extend into Lolo, Flathead, Helena, and the Lewis and Clark National Forests. All three wildernesses total 1,535,352 acres (6,070 km²). As directed by the creation of the Wilderness Act in 1964, "no roads or structures were to be built, vehicles and other mechanical equipment were not to be used." The Bob is the second largest wilderness in the lower 48 states after the Frank Church River of No Return Wilderness in Idaho. The five ranger districts administering the Bob manage 1,856 miles (2,970 km) of trail that are open to foot and stock use only.

Wilderness areas do not allow motorized or mechanical equipment including bicycles or hanggliders. Although camping and fishing are allowed with proper permit, no roads or buildings are constructed and there is also no logging or mining, in compliance with the 1964 Wilderness Act. Some administrative cabins constructed in the early 1920s afford refuge for trails crews and wilderness rangers. Wilderness areas within National Forests and Bureau of Land Management areas also allow hunting in season.

"The Bob", as it is known by locals and nicknamed by the U.S. Forest Service employees, ranges in altitudes of 4,000 to over 9,000 feet (1,220 to 2750 m). A long escarpment known as the Chinese wall averages 1,000 feet (300 m) high from its base and extends for 40 miles (64 km). Aside from numerous waterfalls, lakes and dense forests, the wilderness is also prime Grizzly bear habitat and the U.S. Forest Service claims that the population density of this species is higher than can be found anywhere else in the U.S. outside of Alaska. The Bob is also home to many other large mammals such as moose, elk, black bear, mountain goat, bighorn sheep, wolverine, mountain lion, lynx and wolf. Bald eagles, osprey, pelican and trumpeter swan are just a few of the bird species found. The dense old growth forests are dominated by douglas fir, larch and spruce.

The wilderness can be accessed by trails from surrounding roads. U.S. highway 2 is to the north and separates the Bob Marshall Wilderness Complex from Glacier National Park (U.S.). U.S. 89 and 287 are to the east and Montana highway 200 and 83 are to the south and west. The wilderness is approximately 75 miles (120 km) west of Great Falls, Montana, 50 miles (80 km) north of Missoula, Montana and 30 miles (48 km) east of Kalispell, Montana.