The Mountain States (also known as the Mountain West and the Interior West) form one of the nine geographic divisions of the United States that are officially recognized by the United States Census Bureau. It is a subregion of the Western United States. The Mountain States are usually split up into two other regions known as the Northwest and Southwest. Idaho, Montana, and Wyoming are considered part of the northwest, while Arizona, New Mexico, Colorado, Nevada, and Utah are considered part of the southwest.

The division consists of eight states: Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming. These eight states have the highest mean elevations of all 50 U.S. states. Together with the Pacific States of Alaska, California, Hawaii, Oregon and Washington, 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 Montana, Idaho, Wyoming, parts of Utah, Colorado, and New Mexico. Arizona and Nevada, as well as most other parts of Utah and New Mexico have other mountain ranges and scattered mountains located in them as well.