Plesiosaurus was the first plesiosaur discovered and named. For a long time Plesiosaurus was treated as waste-basket taxon. This means that many different specimens were allocated to the genus Plesiosaurus under different species names, even when they were very different. By modern standards, many of these specimens deserve a generic name of their own, or may be of dubious validity entirely. The lumping of many species into the genus Plesiosaurus was initiated during the 1800s when plesiosaurs were first being studied and described. Many species formerly included in Plesiosaurus are now renamed and most of them do not even belong in the family Plesiosauridae. For example, ‘Plesiosaurus’ rostratus was renamed Archaeonectrus, ‘Plesiosaurus’ conybeari was renamed Attenborosaurus.
Storrs (1997) reduced the number of valid species of Plesiosaurus to three. However, two of those have unique features that warrant generic separation: ‘Plesiosaurus’ guilielmiiperatoris is today regarded as Seeleyosaurus, a name reinstated by Grossman (2007), and ‘Plesiosaurus’ brachypterygius is now known by the name of Hydrorion (Grossman, 2007). All this means that at present, Plesiosaurus contains only the single valid species, P. dolichodeirus. Some species arbitrarily referred to Plesiosaurus still remain today, pending revision. ‘Plesiosaurus’ macrocephalus, for example, is possibly a juvenile rhomaleosaurid.
Plesiosaurus belongs to the family Plesiosauridae and was regarded as the sole member of the group for several years. However, recent research into early Jurassic plesiosaurs has revealed a greater generic diversity of plesiosaurids.
The holotype specimen of P. dolichodeirus (Conybeare 1824) is NHMUK 22656, a complete skeleton.