With a land area of 145,552 mi² (376,978 km²) the state of Montana is the fourth largest in the United States(after Alaska, Texas, and California). To the north, Montana and Canada share a 545-mile(877 km) border. The state borders the Canadian provinces of British Columbia, Alberta, and Saskatchewan, more provinces than any other state.

To the east, the state borders North Dakota and part of South Dakota. To the south is Wyoming and to the west and southwest is Idaho. The topography of the state is diverse, but roughly defined by the Continental Divide, which runs on an approximate diagonal through the state from northwest to south-central, splitting it into two distinct eastern and western regions.

Montana is well known for its mountainous western region, part of the northern Rocky Mountains. However, about 60% of the state is actually prairie, part of the northern Great Plains. Nonetheless, even east of the Continental Divide and the Rocky Mountain Front, there are a number of isolated "Island Ranges" that dot the prairie landscape.

The Bitterroot Mountains divide the state from Idaho to the west with the southern third of the range blending into the Continental Divide. Mountain ranges between the Bitterroots and the top of the Continental Divide include the Cabinet Mountains, the Missions, the Garnet, Sapphire, Flint Creek, and Pintlar ranges.