Megalonyx, meaning “Great claw,” is an extinct genus of giant ground sloths in the family Megalonychidae. The genus was endemic to North America from the Hemphillian stage of the Late Miocene to the Rancholabrean stage of the Pleistocene epoch (10.3 million to 11,000 years ago).

The species, M. leptostomus, was named in 1893. It lived from Florida to Texas, north to Kansas and Nebraska, and west to New Mexico, Nevada, Oregon, and Washington. Fossils have been discovered in numerous sites within its range. Excavations at Tarkio Valley in Iowa may reveal hints to Megalonyx’s social structure. An adult was found in direct association with two juveniles of different ages, suggesting that adults cared for their young of different generations.

Its generic name, Megalonyx, was proposed by Thomas Jefferson in 1797, based on fossil specimens of what later came to be called M. jeffersonii that he received from western Virginia. His presentation to the American Philosophical Society that year is often credited as the beginning of vertebrate paleontology in North America. Megalonyx was not formally accepted as the generic name until 1825 when American naturalist Richard Harlan adopted the term.

The closest living relatives of Megalonyx are those from the two-toed sloth genus, Choloepus.