Plesiopleurodon is an extinct genus of plesiosauroid from the Late Cretaceous of North America. It was named by Carpenter based upon a complete skull and lower jaws, neck vertebrae, and a right coracoid. It was collected from the Belle Fourche Shale (lower Cenomanian), in the Rattlesnake Hills of Wyoming. In naming the specimen, Carpenter (1996, p. 264) noted "Of all known pliosaurs, Plesiopleurodon wellesi most closely resembles Liopleurodon ferox from the Oxfordian of Europe, hence the generic reference."
The species is characterized by a moderately long symphysis bearing 8 pairs of teeth, teeth that are nearly circular in cross-section and which are smooth on the outer surface (except near the base), ribs of the neck vertebrae being singled-headed (double-headed in Jurassic pliosaurs), and a long slender interpectoral bar on the coracoid.