Liopleurodon is a genus of Pliosaurs, large, carnivorous marine reptiles which lived during the mid to late Jurassic period (c. 160 million to 155 million years ago). The largest and best-known species is undoubtedly Liopleurodon ferox, first identified by H.E Sauvage in 1873.

Four strong paddle-like limbs suggest that Liopleurodon was a powerful swimmer. Its 4-flipper motion has reportedly died with the extinction of the liopleurodon. Studies of the skull have showed that it could scan the water with its nostrils to ascertain where certain smells came from. Liopleurodon was omnivorous and it is unlikely it had many, if any, predators.

Fossils of the creature have been found mainly in Germany and the United Kingdom from the Jurassic period, when Europe was covered by a large sea. The issue of its maximum size has been somewhat controversial. Most fossil evidence of Liopleurodon ferox seems to indicate that these beasts grew from 7 to 10 metres long; however, as with its relative Kronosaurus, there is some uncertainty whether current reconstructions are correct. Fossil evidence from Great Britain indicates much larger contemporary pliosaurs, up to 15 metres or even longer, but the evidence is too fragmentary to determine whether it belonged to Liopleurodon or to a species from some other genus.

In 2002 the discovery of a very large pliosaur in Mexico was announced. This came to be known as the 'Monster of Aramberri'. Conservative estimates gave a length of at least 15 metres, despite the possibility of its being a juvenile specimen. However, although widely reported as such, it did not belong to the Liopleurodon genus. Estimates of maximum size (most likely exaggerated) had already been circulated in the 1999 documentary series Walking with Dinosaurs where an enormous pliosaur was presented as a 25-meter-long Liopleurodon. However, most paleontologists believe that Liopleurodon (or any other pliosaur) could not have grown that large.