Plesiopleurodon is an extinct genus of Mesozoic marine reptiles, belonging to the Sauropterygia, known from the Late Cretaceous of North America. It was named by Kenneth Carpenter based on a complete skull with a mandible, cervical vertebra, and a coracoid. In naming the specimen, Carpenter noted "Of all known pliosauroids, Plesiopleurodon wellesi most closely resembles Liopleurodon ferox from the Oxfordian of Europe, hence the generic reference." It was initially described as a pliosauroid due to it short neck, a common trait of the superfamily. However, later exploration into the relationships of both groups indicate that not all pliosauroids have short necks and not all plesiosauroids have long necks. Later research indicates it is a member of the Polycotylidae, within the clade Occultonectia.