Gallimimus is a genus of theropod dinosaur that lived during the Late Cretaceous period, about seventy million years ago.

Despite its name, which is Greek for "chicken mimic", Gallimimus weighed more than 900 pounds. A better comparison might be a beefy, low-to-the-ground, aerodynamic ostrich.

Gallimimus was discovered in 1964 by Zofia Kielan-Jaworowska in the Gobi Desert of Mongolia.

Ornithomimidae (meaning "bird-mimics") is a family of theropod dinosaurs which bore a superficial resemblance to modern ostriches. The skulls of ornithomimids were small, with large eyes, above relatively long and slender necks. All had toothless beaks.

The adult holotype (specimen IGM 100/11) was about 6 metres (20 ft) long and 1.9 metres (6.2 ft) tall at the hip.

Gallimimus was toothless with a keratinous (horny) beak and a delicate lower jaw.

At first scientists thought they were herbivores, but it is now believed that Gallimimus had an opportunistic, omnivorous diet like seagulls, eating small animals like insects and lizards, eggs, and some plant material.

As an ornithomimid, Gallimimus would have been a fleet (or cursorial) animal, using its speed to escape predators. Its top speed has been estimated at 34 mph (56 km/h).

Dr. Alan Grant and the Murphy children witness a Gallimimus flock in its enclosure fleeing from Jurassic Park's Tyrannosaurus rex. The T. rex manages to kill one unlucky specimen who trips on another Gallimimus when the flock is running for safety.

Like birds today, Gallimimus had feathers, wing-like structures which may have been used for courtship, display, and brooding, and hollow chambers in its bones, which would have been filled with air sacs in life.