Megalonyx (Greek, "great-claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America.

[4] Megalonyx is suggested to have descended from Pliometanastes, a genus of ground sloth that had arrived in North America during the Late Miocene around 9 million years ago, prior to the main phase of the Great American Interchange.

Megalonyx is notable for having been originally described by future U.S. President Thomas Jefferson in 1799 based on remains found in West Virginia; the species M. jeffersonii was described later, named in honor of him.

He asked Lewis and Clark, as they planned their famous expedition in 1804–1806, to keep an eye out for living specimens of Megalonyx, as this would support his case.

Richard Harlan in 1825 revived the genus Megalonyx with the type species M. jeffersonii, and provided additional taxonomic description.

A 2024 review paper by Loren E. Babcock found that Simpson's views were mistaken, and clarified that Jefferson was the valid author of the genus, as the description was done in accordance with the rules of taxonomic nomenclature at the time, despite him not assigning a species to it.

[1] Recent research confirms that the sloth bones were discovered in Haynes Cave in Monroe County, West Virginia.

For many decades in the twentieth century, the reported origin of Jefferson's "Certain Bones" was Organ Cave in what is now Greenbrier County, West Virginia.

Smithsonian paleontologist Frederick Grady presented evidence in 1995 confirming Haynes Cave as the original source of Jefferson's fossil.

This species lived from Florida to Texas, north to Kansas and Nebraska, and west to New Mexico, Nevada, Oregon, and Washington.

Based on molecular results, its closest living relatives are the three-toed sloths (Bradypus); earlier morphological investigations came to a different conclusion.

[14][15] Megalonyx is thought to be descended from Pliometanastes, a ground sloth that arrived in North America during the late Miocene, around 9 million years ago.

[24] In 2010, a specimen was discovered at the Ziegler Reservoir site near Snowmass Village, Colorado, in the Rocky Mountains at an elevation of 8,874 feet (2,705 m).

In Ohio, a specimen of Megalonyx jeffersonii, dubbed the "Firelands Ground Sloth", dating to around 13,738 to 13,435 calibrated years Before Present (~11,788 to 11,485 BCE) was found with cut marks indicative of butchery, suggesting that hunting may have played a role in its extinction.