Deinonychus is a genus of dromaeosaurid theropod dinosaur that flourished in western North America during the Early Cretaceous Period, about 115 to 108 million years ago.

Deinonychus is derived from the Greek words deinós (meaning "terrible") and ónuchos (meaning "claw"). The name was inspired by this dinosaur's principal weapons, which were large sicklelike talons 13 cm (5 inches) long on the second toe of each foot.

Geological evidence suggests that Deinonychus inhabited a floodplain or swamplike habitat. The paleoenvironment of both the upper Cloverly Formation and the Antlers Formation, in which remains of Deinonychus have been found, consisted of tropical or sub-tropical forests, deltas and lagoons, not unlike today's Louisiana.

Paleontologist John Ostrom's study of Deinonychus in the late 1960s revolutionized the way scientists thought about dinosaurs, leading to the "dinosaur renaissance" and igniting the debate on whether dinosaurs were warm-blooded or cold-blooded. Before this, the popular conception of dinosaurs had been one of plodding, reptilian giants. Ostrom noted the small body, sleek, horizontal posture, ratite-like spine, and especially the enlarged raptorial claws on the feet, which suggested an active, agile predator.

Velociraptors are well known as vicious and cunning killers thanks to their portrayal in Jurassic Park, but the "raptors" portrayed in the films and novels were actually modeled after Deinonychus. Author Michael Crichton met with John Ostrom several times at Yale University to discuss details of the animal's possible range of behaviors and appearance, and at one point apologetically told Ostrom that he had decided to use the name Velociraptor in place of Deinonychus because it was "more dramatic." According to Ostrom, Crichton stated that the Velociraptor of the novel was based on Deinonychus in almost every detail, and that only the name had been changed.

North American fossil sites have found a lot of individual Deinonychus specimens of varied ages in concentrated sites. Also, they have been found in close proximity to a possible prey animal, called Tenontosaurus. Deinonychus was about 1/20th the size of Tenontosaurus, however, so how did it bring the larger dinosaur down? Some paleontologists have speculated that this is an indication of pack hunting.

The first remains were uncovered in 1931 in southern Montana near the town of Billings. The team leader, paleontologist Barnum Brown, was primarily concerned with the excavation of the ornithopod dinosaur Tenontosaurus, but in his field report from the dig site, he reported the discovery of a small carnivorous dinosaur close to a Tenontosaurus skeleton, "but encased in lime difficult to prepare." He informally called the animal "Daptosaurus agilis" and made preparations for describing it and having the skeleton, specimen AMNH 3015, put on display, but never finished this work.

Ostrom noted similarities between the forefeet of Deinonychus and that of birds, an observation which led him to revive the hypothesis that birds are descended from dinosaurs. Forty years later, this idea is almost universally accepted.

While no skin impressions have ever been found in association with fossils of Deinonychus, several closely related dinosaurs, including Microraptor and Velociraptor, have been found with feather impressions and quill knobs, leading researchers to believe that the Dromaeosauridae, including Deinonychus, had feathers.

The "raptor prey restraint" (RPR) model of predation, proposes that Deinonychus killed its prey in a manner very similar to extant accipitrid birds of prey: by leaping onto its quarry, pinning it under its body weight, and gripping it tightly with the large, sickle-shaped claws. Like accipitrids, the dromaeosaur would then begin to feed on the animal while still alive, until it eventually died from blood loss and organ failure.