Mantellisaurus is a genus of iguanodontian dinosaur that lived in the Barremian and early Aptian ages of the Early Cretaceous Period of Europe. Its remains are known from Belgium (Bernissart), England, Spain and Germany. The type and only species is M. atherfieldensis. Formerly known as Iguanodon atherfieldensis, the new genus Mantellisaurus was erected for the species by Gregory Paul in 2007. According to Paul, Mantellisaurus was more lightly built than Iguanodon and more closely related to Ouranosaurus, making Iguanodon in its traditional sense paraphyletic. It is known from many complete and almost complete skeletons. The genus name honours Gideon Mantell, the discoverer of Iguanodon.

The Sainte-Barbe Clays Formation is a geological formation in Belgium. It is found in localised areas of the northern margin of the Mons Basin, alongside the equivalently aged Hautrage and Baudour Clay Formations. It is Upper Barremian-Lower Aptian in age. It predominantly consists of laminated clay, with some lignite. It is well known for the "Iguanodon sinkhole" locality near Bernissart where many specimens of Iguanodon bernissartensis were described by Louis Dollo in the late 19th century.