Nothronychus mckinleyi is a dinosaur classify in the group Therizinosauria, strange theropods with a toothless beak, a bird-like hip (like the non-related ornithischians), and four-toed feet, with all four toes facing onward. The type species of this dinosaur was found by James I. Kirkland and Douglas G. Wolfe in 2001 near New Mexico's border with Arizona, in an area recognized as the Zuni Basin.

The name Nothronychus, meaning "sloth-like claw", was selected because the animal reminded Kirkland of a giant earth sloth. It is the first example of therizinosaurs to be established in the Americas. Previous discoveries had been from China and Mongolia. Nothronychus was better than Erlikosaurus or Segnosaurus, but in some ways more prehistoric than these Asian relatives. A related dinosaur from a considerably earlier period, Falcarius utahensis, was exposed in Utah in 2005.

The sloth-like Nothronychus lived 90 million years past, during the Mid Cretaceous period of the Mesozoic era, in swampy forests alike to modern bayous in Louisiana. The era was a time of extreme global warming, with ocean levels 300 m (1,000 ft) higher than current levels and a significantly abridged amount of dry land. Almost no dinosaur fossils have been found from this time, chiefly in North America, making this and associated discoveries very important.

Nothronychus is a associate of the theropod group of carnivorous dinosaurs that includes Tyrannosaurus. But Nothronychus, and its close relations, evolved into plant-eaters. It was bipedal and walked more upright than its carnivore ancestors. The person weighed about a tonne, was 4.5-6 m (15-20 ft) long and stood 3-3.6 m (10-12 ft) tall

A rebuilding of 40 to 50 percent of its skeleton, from two animals, allows scientists to describe the dinosaur as having:

Small head with a mouth full of leaf-shaped teeth intended for shredding vegetation
Long, thin neck

Long arms with dexterous hands and 10 cm (4 in) bent claws on its fingers
Large abdomen
Stout back legs
Relatively short tail

Bird-like characteristics, and potted feathers in fossils of its Asian relatives, indicate Nothronychus may have had a downy coat of feathers, perhaps resembling an emus. No feather imitations were established with the fossil remains, almost certainly because the surrounding sediment was unable to protect such delicate features.

The Arizona Republic newspaper, first announced the detection on 19 June 2001, and the dinosaur's name in a column by R. E. Molnar. Nothronychus was formally described in the Journal of Vertebrate Paleontology on 22 August 2001.