The Rocky Mountain locust (Melanoplus spretus) is an extinct species of grasshopper that ranged through the western half of the United States and some western portions of Canada with large numbers seen until the end of the 19th century. Sightings often placed their swarms in numbers far larger than any other locust species, with one famous sighting in 1875 estimated at 198,000 square miles (510,000 km²) in size (greater than the area of California), weighing 27.5 million tons and consisting of some 12.5 trillion insects, the greatest concentration of animals ever speculatively guessed, according to Guinness World Records.

Less than 30 years later, the species was apparently extinct. The last recorded sighting of a live specimen was in 1902 in southern Canada. Because a creature so ubiquitous was not expected to become extinct, very few specimens were ever collected (though a few preserved remains have been found in Knife Point Glacier, Wyoming and Grasshopper Glacier, Montana).

Rocky Mountain locusts were a part of the diet of the critically endangered or possibly extinct Eskimo curlew (Numenius borealis) on its spring migration and the extinction of the locust has been speculated as being a factor in the decline of the curlew.