desert area in southwestern Arizona, U.S., at the Mexican border 12 miles (19 km) south of Ajo. Established in 1937 with an area of 517 square miles (1,339 square km), it preserves segments of the mountainous Sonoran Desert.

The monument is named for the organ-pipe cactus, which is found in the United States only in this locality and is so called because its branches resemble organ pipes. Desert ironwood, ocotillo, saguaro, creosote bush, and numerous other desert plants are also found there. Typical species of wildlife are bighorn sheep, javelinas, coyotes, a variety of birds, and the venomous Gila (Gila monster) monster. Portions of El Camino del Diablo (“Devil's Highway”), the historic Spanish route along which hundreds of miners and pioneers lost their lives, may still be seen.