Regnosaurus (meaning "Sussex lizard") is a genus of herbivorous stegosaurian dinosaur that lived during the Early Cretaceous Period in what is now England.

The fossil remains, a portion of the right lower jaw, were found near Cuckfield in Sussex, and made part of the collection of the British Museum of Natural History.

In 1848, after several real jaws of Iguanodon had been discovered, Mantell changed his position, concluding it was a related but different genus or subgenus, coining the name Regnosaurus Northamptoni.

Regnosaurus is known only from the holotype BMNH 2422, a right mandibular (lower jaw) fragment, consisting of a third of the dentary and a part of the splenial, found in the Tunbridge Wells Sand Formation.

Other bone fragments have sometimes been referred to Regnosaurus, such as a fossil pubis recovered on the Isle of Wight, but as these are of other parts of the body and a reasonably complete skeleton is lacking, the identity cannot be proven.