Cryptosaurus (meaning "hidden lizard")[1] is a dubious genus of dinosaur known from a partial femur from the Late Jurassic of England.

The description of 1869, part of a catalogue of the Cambridge University collection, was rather minimal, consisting of the single sentence "On Shelf g is temporarily placed the femur of a Dinosaur from the Oxford Clay, Cryptosaurus eumerus".

[3] The holotype, CAMSM J.46882,[4] was discovered in the Great Gransden brick pit, in strata of the Ampthill Clay Formation, dating from the upper Oxfordian.

Seeley thought that Cryptosaurus was a relative of Iguanodon, the first such discovered from the Oxford Clay — hence the generic name.

[7] This was in error, a mistake originally caused by a wrong spelling of Cystosaurus in the French Bibliothèque universelle des sciences, belles-lettres, et arts, and the name Cryptosaurus has precedence.