The Western United States, commonly referred to as the American West or simply "the West," traditionally refers to the region comprising the westernmost states of the United States. Because the U.S. expanded westward after its founding, the meaning of the West has evolved over time. Prior to about 1800, the crest of the Appalachian Mountains was seen as the western frontier. Since then, the frontier moved further west and the Mississippi River was referenced as the easternmost possible boundary of the West.

In the 21st century, the states which include the Rocky Mountains and the Great Basin to the West Coast are generally considered to comprise the American West.

Besides being a purely geographical designation, "The West" also has anthropological connotations. While this region has its own internal diversity, there is arguably an overall shared history, culture (music, cuisine), mind set or world view and closely interrelated dialects of English. As with any region of such geographically large extent and varied culural histories, many subregions of The American West possess distinguishing and idiosyncratic qualities.

The "West" had played an important part in American history; the Old West is embedded in America's folklore.

In its most extensive definition, the western U.S. is the largest region, covering more than half the land area of the United States. It is also the most geographically diverse, incorporating geographic regions such as the Pacific Coast, the temperate rainforests of the Northwest, the Rocky Mountains, the Great Plains, most of the tall-grass prairie eastward to Western Wisconsin, Illinois, the western Ozark Plateau, the western portions of the southern forests, the Gulf Coast, and all of the desert areas located in the United States (the Mojave, Sonoran, Great Basin, and Chihuahua deserts).