Custer National Forest is located primarily in the south central part of the U.S. state of Montana but also has separate sections in northwestern South Dakota. With a total area of , the forest comprises over 10 separate sections. While in the westernmost sections, Custer National Forest is a part of the Greater Yellowstone Ecosystem, the easternmost sections are a combination of forest "islands" and grasslands. A portion of the forest is also part of the Absaroka-Beartooth Wilderness and constitutes over a third of the wilderness land. South of Red Lodge, Montana, the Beartooth Highway (U.S. 212) passes through the forest en route to Yellowstone National Park.