Liopleurodon (/ˌlaɪoʊˈplʊərədɒn/; meaning 'smooth-sided teeth') is a genus of large, carnivorous marine reptile belonging to the Pliosauroidea, a clade of short-necked plesiosaurs. The two species of Liopleurodon lived during the Callovian stage of the Middle Jurassic Period (c. 160 to 155 mya). It was the apex predator of the Middle to Late Jurassic seas that covered Europe. The largest species, L. ferox, is estimated to have grown up to 6.39 meters (21.0 ft) in length.
The name "Liopleurodon" (meaning "smooth-sided tooth") derives from Ancient Greek words: λεῖος [leios], "smooth"; pleurá, side or rib; and odṓn, tooth.