The Mountain States form one of the nine geographic divisions within the United States which are officially recognized by that country's census bureau.

The division consists of eight states: Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming. Together with the Pacific States of Alaska, California, Hawaii, Oregon and Washington state, the Mountain States constitute the broader region of the West, one of the four regions the United States Census Bureau formally recognizes (the Northeast, South and Midwest being the other three). The word "Mountain" refers to the Rocky Mountains, which run north-south throughout the division, and also to Mountain Standard Time, which all of the division save Nevada in its entirety and the Idaho panhandle observe; in addition, Arizona is one hour behind the other Mountain States (but for the aforementioned exceptions) from the first Sunday in April until the last Sunday in October, because Daylight Saving Time is not used in Arizona.

Since the late 1960s, the Mountain States have moved to challenge the Southern States for the distinction of being the nation's most politically conservative geographical entity; a large part of this trend toward conservatism has been caused by the arrival of many persons who have departed from the more liberal Pacific states, especially California, from that time onward. The brand of conservatism espoused by some of these West Coast transplants has been particularly extreme, as many Neo-Nazi groups have established headquarters in parts of Idaho and Montana.

In their geopolitical book The Day America Told The Truth, James Patterson and Peter Kim place most of the territory found within the Mountain States in a moral region they label Marlboro Country, with the division's eastern and southern salients being slotted into their Granary and L.A.-Mex regions respectively.