The Mountain States (also known as the Mountain West or the Interior West) form one of the nine geographic subregions 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 usually include Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming. The total land area of the eight states is 855,767 square miles (2,216,426 square kilometers).
The words "Mountain States" generally refer to the U.S. states that cover the Rocky Mountains, which are oriented north-south through parts of Montana, Idaho, Wyoming, Colorado, Utah, and New Mexico.
Arizona and Nevada, as well as other parts of Utah and New Mexico, have other smaller mountain ranges and scattered mountains located in them as well. Sometimes, the Trans-Pecos area of the far west of Texas is considered part of the region.