Mantellisaurus is a genus of dinosaur from the Early Cretaceous Period and lived in what is now England. It was discovered by Reginald Hooley in 1917 and was formerly known as a species of Iguanodon, but eventually given its own genus by Gregory Paul in 2007. Paul stated that it was more closely related to Ouranosaurus than Iguanodon. It is known from many partial and nearly complete skeletons. The genus name honors Gideon Mantell, the discoverer of the Iguanodon.
Mantellisaurus was smaller than the Iguanodon, about 1,500 pounds in weight. It had much smaller limbs than Iguanodon and its forelimbs were about half the length of the hind limbs. This dinosaur would have most likely been bipedal when walking or running based on its small frame. It may, however, have stood on all fours when standing still or moving slowly.