Dryosaurus ("oak lizard", due to the vague oak shape of its cheek teeth) was an ornithopod dinosaur livelihood in the Late Jurassic Period. Fossils have been found in the western United States and Tanzania, and were first exposed in the late 19th century.

Dryosaurus had a long neck, long, slim legs, and a long, stiff tail. Its arms, however, with five fingers each, were short. It was concerning eight to 14 feet long and five feet tall (at the hips), and weighed 170 to 200 pounds. Its eyes were fairly large, leading many to believe that it obsessed excellent eyesight.

Dryosaurus had a horny beak and cheek teeth, and like other ornithopods, was an herbivore. Some scientists propose that it stored food in its cheeks. It was almost certainly a herd animal which raised and protected its young after hatching.

A quick and agile runner with physically powerful legs, Dryosaurus used its tail as a counter-balance while creation sharp turns and probably relied on its speed as a main protection against carnivorous dinosaurs. It appears to have lived in forests.

Its intelligence, as deliberate by its Encephalization proportion (brain to body ratio), was midway when compared to other dinosaurs.