Southwest, The, a region of the United States composed of the states of Texas, Oklahoma, New Mexico, and Arizona. It was one of the first regions in what is now the United States to become inhabited, having been the site of human settlement some 20,000 years ago. Except for Texas, however, it remained sparsely populated until late in the 19th century; Oklahoma, New Mexico, and Arizona were not admitted to the Union until the 20th century. (For detailed histories, see articles on the four individual states.)

The Southwest is a spacious region, in parts sparsely settled. It covers approximately one-sixth of the land area of the United States and contains about one-tenth of the population. The Southwest has a wide variety of terrain, including deserts, plains, river valleys, mountains, and canyons. The climate is dry and warm. The economy is centered around mineral resources, particularly petroleum; livestock raising; farming; and tourism.

The Southwest bears the imprint of three distinctive cultures—Indian, Spanish, and Anglo-American. The region was inhabited by various Indian tribes when the first European explorers, the Spanish, entered the area early in the 16th century. All of the Southwest, except Oklahoma, which the United States acquired from France in the Louisiana Purchase of 1803, came under Spanish control and later passed to Mexico. As a result of the Mexican War (1846–48) and the Gadsden Purchase (1853), the United States gained possession of the entire area by the mid-19th century.

American settlers had only begun arriving in the Southwest in the early 19th century. (White settlement was prohibited in Oklahoma, then the Indian Territory, until 1889.) The second half of the 19th century produced the “Old West” of legend, with its cowboys, cattle drives, and homesteaders. Discovery of oil around the turn of the century brought wealth and growth. By the second half of the 20th century, the Southwest was one of the most rapidly developing areas in the nation.