Megalonyx (Greek, "great-claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America. It evolved during the Pliocene Epoch and became extinct at the end of the Late Pleistocene, living from ~5 million to ~13,000 years ago. The type species, M. jeffersonii (also called Jefferson's ground sloth), the youngest and largest known species, measured about 3 m (9.8 ft) in length and weighed up to nearly 1,300 kg (2,900 lb).

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 had the widest distribution of any North American ground sloth, having a range encompassing most of the contiguous United States, extending as far north as Alaska during warm interglacial periods.

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.

Megalonyx became extinct as part of the end-Pleistocene extinction event, simultaneously with all other mainland ground sloths and most other large mammals native to the Americas. These extinctions followed the arrival of humans in the Americas, and there is evidence that humans interacted with Megalonyx, including butchering its remains shortly prior to its extinction.

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