Liopleurodon (ˌlaɪoʊ-ˈplʊərədɒn; meaning 'smooth-sided teeth') is an extinct genus of large, carnivorous marine reptile belonging to the Thalassophonea, a clade of short-necked pliosaurid plesiosaurs. Liopleurodon lived from the Callovian Stage of the Middle Jurassic to the Kimmeridgian stage of the Late Jurassic Period (c. 166 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 in length based on the largest known skull.
The name "Liopleurodon" (meaning "smooth-sided tooth") derives from Ancient Greek words: λεῖος leios, "smooth"; πλευρά pleurá, "side" or "rib"; and ὀδόν odṓn, "tooth".
Even before Liopleurodon was named, material likely belonging to it was described. In 1841, Hermann von Meyer named the species Thaumatosaurus oolithicus based on a fragmentary specimen consisting of partial teeth, skull elements, vertebrae, and ribs from deposits in Württemberg, Germany, possibly dating to the Oxfordian. However, this material is nondiagnostic, lacking distinguishing features. Johann Andreas Wagner published a description of a large plesiosaur tooth from Bavaria, Germany, in 1852, assigning it to a new species that he named Pliosaurus giganteus. However, in 1824, William Conybeare had named a species of Plesiosaurus, Plesiosaurus giganteus, and this species was later viewed as a synonym of either Pliosaurus brachydeirus or P. brachyspondylus by following authors. Since the name Pliosaurus giganteus had been used prior to Wagner's publication, Wagner's name is invalid due to preoccupation. In 1838, Hermann von Meyer applied the name Ischyrodon meriani to a large tooth from Oxfordian-aged rocks in Fricktal, Switzerland. This tooth lacks identifying characteristics, and therefore it is not clear what it belonged to, although Lambert Beverly Tarlo noted the possibility of it pertaining to Liopleurodon in 1960.