The Southwestern United States, commonly referred to as the Southwest, is a region of the western United States. The USGS does not define regions so regional definitions vary from source to source. The Southwest always includes New Mexico and Arizona, but often takes in western Texas, the southern portions of Nevada, Utah, and Colorado, western Oklahoma, and southeastern California.

The population density of the region is less than three people per square mile. The Southwest is also more ethnically varied than neighboring regions, with significant Spanish American, Mexican American, and American Indian populations.

The Southwest contains many large cities and metropolitan areas of the country, despite the low population density of the region as a whole.. Los Angeles is commonly mistaken for being part of the region though it it actually just considered either Western United States or the Pacifc region of the United States. Phoenix, Dallas, San Antonio, San Diego and Houston all rank among the top ten most populous cities in the South West. Fort Worth and Austin, and are in the top 20. Many of the states in this region, such as Nevada and Arizona, have witnessed some of the highest population growth in the United States. Urban areas in this region, like Las Vegas and Phoenix are some of the fastest growing cities in the country.

Tex-Mex cuisine was popularized in San Antonio, and has spread to many other areas, both in Texas and in other nearby western states. New Mexican cuisine, includes a notable focus on its respective regional emphasis on chile spice. Arizona's very popular Arizona-Sonoran cuisine, holds its own meld of northern Mexican states' love of heavy sauces, as well as that regional respect for chile heat. And, the San Francisco burrito and "Fresh Mex" or Baja California-style of Mexican food is also commonly associated with the Southwest.