The type specimen (WM 851.S, skull and post-cranial skeleton) of Plesiosaurus propinquus Tate & Blake, 1876, from the Toarcian (Upper Lias, Lower Jurassic) of Yorkshire, is re-examined. New observations permit a detailed understanding of the cranial anatomy of this specimen and hence its taxonomic affinity. The few differences between Plesiosaurus propinquus and Rhomaleosaurus zetlandicus cannot be considered diagnostic. Consequently, Plesiosaurus propinquus is here assigned to and considered a junior synonym of Rhomaleosaurus zetlandicus. Currently only three valid species of the genus Rhomaleosaurus are known; a complete revision of the specimens formerly attributed to Rhomaleosaurus must now be carried out.