The Ornithischia are a group of medium to large plant-eating dinosaurs belonging to the life-form group called Dinosauria. They include one of the earliest discovered dinosaurs, Iguanodon, as well as the famous crested and "duck-billed" hadrosaurs. Several of these are noted for the spacious and bizarrely shaped sinus regions in their skulls. Ornithischia is an extinct order of beaked, herbivorous dinosaurs. The name "Ornithischia" means "bird-hipped," and they had a pelvis that superficially resembled a bird's pelvis in which the pubis points backwards. However, birds are more closely related to the Saurischia, or "lizard-hipped" dinosaurs, than to the ornithischian dinosaurs. Ornithischia includes a variety of species which were primarily herbivores.

Some commonly known Ornithischians include the horned dinosaurs (ceratopsians), armoured dinosaurs (stegosaurs and ankylosaurs), Iguanodon, and the "duck-billed" dinosaurs (hadrosaurids). Being herbivores that sometimes lived in herds, they were more numerous than the saurischians. Many were prey animals for the theropods and were usually smaller than the sauropods.