Liopleurodon (meaning 'smooth-sided teeth') is a genus of large, carnivorous (meat eater) marine reptile. It was the apex predator of the Middle to Late Jurassic seas that covered Europe. The largest species is estimated to have grown up to 6.4 metres (21 ft) in length.

Four strong paddle-like limbs suggest that Liopleurodon was a powerful swimmer. A study involving a swimming robot has demonstrated that although this form of propulsion is not especially efficient, it provides very good acceleration—a desirable trait in an ambush predator. Studies of the skull have shown that it could probably scan the water with its nostrils to ascertain the source of certain smells.