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 holotype fossil, NHMUK PV R5764, was originally discovered by Reginald Walter Hooley in 1914 in the upper Vectis Formation of southern England and reported upon in 1917. He posthumously named it Iguanodon atherfieldensis in 1925. Atherfield is the name of a village on the southwest shore of the Isle of Wight where the fossil was found.

The Maidstone specimen was discovered in a quarry in Maidstone, Kent, owned by William Harding Bensted, in February 1834 (lower Lower Greensand Formation). In June 1834 it was acquired for £25 by scientist Gideon Mantell. He was led to identify it as an Iguanodon based on its distinctive teeth. The Maidstone slab was utilized in the first skeletal reconstructions and artistic renderings of Iguanodon, but due to its incompleteness, Mantell made some mistakes, the most famous of which was the placement of what he thought was a horn on the nose.

Shortly after the discovery, tension began to build between Mantell and Richard Owen, an ambitious scientist with much better funding and society connections in the turbulent worlds of Reform Act–era British politics and science. Owen, at the time a firm creationist, opposed the early versions of evolutionary science ("transmutationism") then being debated and used what he would soon coin as dinosaurs as a weapon in this conflict. With the paper describing Dinosauria, he scaled down dinosaurs from lengths of over 61 metres (200 ft), determined that they were not simply giant lizards, and put forward that they were advanced and mammal-like, characteristics given to them by God; according to the understanding of the time, they could not have been "transmuted" from reptiles to mammal-like creatures.

In 1849, a few years before his death in 1852, Mantell realised that the genus today known as Mantellodon was not a heavy, pachyderm-like animal, as Owen was putting forward, but had slender forelimbs; however, his passing left him unable to participate in the creation of the Crystal Palace dinosaur sculptures, and so Owen's vision of the dinosaurs became that seen by the public for decades. With Benjamin Waterhouse Hawkins, Owen had nearly two dozen lifesize sculptures of various prehistoric animals built out of concrete sculpted over a steel and brick framework; two Mantellodon, one standing and one resting on its belly, were included. Before the sculpture of the standing Mantellodon was completed, a banquet for twenty was held inside it.

The discovery of much better specimens of Iguanodon bernissartensis in later years revealed that the horn was actually a modified thumb. Still encased in rock, the Maidstone skeleton is currently displayed at the Natural History Museum in London. The borough of Maidstone commemorated this find by adding an Iguanodon as a supporter to their coat of arms in 1949. This specimen has become linked with the name I. mantelli, a species named in 1832 by Christian Erich Hermann von Meyer in place of I. anglicus, but it actually comes from a different formation than the original I. mantelli/I. anglicus material. The Maidstone specimen, also known as Gideon Mantell's "Mantel-piece" and formally labelled as NHMUK 3741, was subsequently excluded from Iguanodon.

It was classified as cf. Mantellisaurus by McDonald (2012), as cf. Mantellisaurus atherfieldensis by David Bruce Norman (2012), and made the holotype of a separate genus and species Mantellodon carpenteri by Gregory S. Paul (2012). The generic name combines Mantell's name with a Greek odon, "tooth", analogous to Iguanodon. The specific name honours Kenneth Carpenter for his work on dinosaurs in general and iguandonts in particular. David Norman, in 2013, considered Paul's description of Mantellodon to be inadequate, identical to that given by Paul of Darwinsaurus and entirely incorrect, noting that no dentary is preserved in the holotype specimen, and that the preserved forelimb elements "are gracile, carpals are not preserved, the metacarpals are elongate and slender, and a thumb spike is not preserved". Norman considered the holotype specimen of Mantellodon carpenteri to be referable to the species Mantellisaurus atherfieldensis.

In 2021, a sculpture nicknamed Iggy the Iguanodon based on the Maidstone specimen was unveiled at the Maidstone East station. The dinosaur is also featured in Maidstone's coat of arms, and is the only dinosaur featured in a borough's coat of arms in the UK.