Mantellisaurus is an iguanodontian dinosaur genus that inhabited Europe during the Barremian and early Aptian stages of the Early Cretaceous Period. Fossils of Mantellisaurus have been discovered in various European countries, including Belgium (Bernissart), England, Spain, and Germany. The species name is M. atherfieldensis, and it was previously classified as Iguanodon atherfieldensis before being reclassified under the new genus Mantellisaurus.

Mantellisaurus was a relatively lightweight iguanodontian dinosaur. It was smaller in size compared to Iguanodon bernissartensis, with an estimated weight of around 750 kilograms (1,650 pounds). Notably, its forelimbs were proportionally shorter than those of I. bernissartensis. In Mantellisaurus, the forelimbs measured approximately half the length of the hindlimbs, while in I. bernissartensis, they were about 70 percent the length of the hindlimbs. Because of its shorter forelimbs and body, some experts proposed that Mantellisaurus primarily walked on two legs, adopting a quadrupedal stance only when stationary or moving slowly.

Footprints found in the same area where the Mantellisaurus holotype fossils were discovered suggest that Mantellisaurus moved in familial groups.