Great Basin, a large, arid region in the western United States consisting of many small valleys broken by mountain ranges. It lies between the Wasatch Range of the Rocky Mountains and the Sierra Nevada and Cascades. The Great Basin covers most of Nevada and parts of Utah, California, Idaho, and Oregon. Its area is about 210,000 square miles (544,000 km2).

There is little rainfall in the Great Basin. Its rivers and lakes have no outlet to the Pacific Ocean. Mountain streams flowing into the valleys evaporate or settle into lakes which, lacking outlets, become salty. The largest of these saline, or brackish, lakes is Great Salt Lake. In the southwestern corner of the Great Basin, Death Valley sinks below sea level. Irrigation has reclaimed some parts of the Great Basin for grazing crops, such as hay and alfalfa. Gold, silver, iron ore, and copper are found in the region.