Which desert forms the border between the southwestern U.S. and Mexico?

The Sonoran Desert forms the desert landscape along much of the southwestern U.S.–Mexico border.

It straddles the international boundary across southern Arizona and southeastern California in the United States, and Sonora, Baja California, and Baja California Sur in Mexico. Most of its area lies in Mexico, making “forms the border” a geographic description rather than a claim that the desert is an official boundary.

The Sonoran is unusually biologically rich for a desert. Its warm winters and two seasonal rainy periods support the iconic saguaro cactus, organ-pipe cactus, palo verde trees, and many species of birds, reptiles, and mammals. This seasonal rainfall pattern distinguishes it from the nearby Mojave, which generally has colder winters and drier summers.

A common mix-up is to identify the Mojave because it is also in California and Arizona. The Chihuahuan Desert lies farther east, while the Great Basin Desert is a colder, higher desert to the north. The Sonoran’s distinctive combination of heat, rainfall, and cactus-rich vegetation makes it the canonical answer.