Ornithomimus, which means "bird mimic," is a genus of ornithomimid theropod dinosaurs that lived during the Campanian and Maastrichtian stages of the Late Cretaceous period in Western North America. Fossil evidence suggests that Ornithomimus was a fast-moving, bipedal dinosaur that possessed feathers and featured a small, toothless beak, possibly indicative of an omnivorous diet.

Similar to other members of the ornithomimid family, Ornithomimus species are recognized for their features such as three weight-bearing toes on their feet, elongated arms, and extended necks culminating in bird-like skulls characterized by toothless beaks. These bipedal dinosaurs bore a resemblance to ratites and were renowned for their agility as runners, thanks to their elongated limbs and hollow bones. Ornithomimus dinosaurs also possessed notably large brains and eyes. Although ornithomimids, in general, had relatively large brains compared to other non-avian dinosaurs, this doesn't necessarily imply a high level of intelligence.

The dietary preferences of Ornithomimus remain a subject of considerable debate. Being theropods, ornithomimids might have been carnivorous, but their physical characteristics make them well-suited for a predominantly herbivorous lifestyle. Proposed items in their potential diet include insects, crustaceans, fruits, leaves, branches, eggs, lizards, and small mammals.

Ornithomimus possessed legs that were evidently adapted for swift movement, with the tibia approximately 20% longer than the femur. The presence of large eye sockets implies sharp visual acuity and raises the possibility that these dinosaurs may have been active during the nighttime hours.