Pachystropheus (After Greek Pachys, "Thick" and Strophaios, Vertebrae) is a genus of prehistoric thalattosaurian marine reptile, from the Rhaetian (Late Triassic) of southwestern England.

[3] However, other authors consider attribution of Pachystropheus to Choristodera problematic, stating that it depends on vertebral and girdle characters that are also found in the skeletons of aquatic reptiles other than choristoderes;[4] most of the diagnostic features of choristoderes are skull features, but the presence of these cannot be confirmed in Pachystropheus, as there is no confirmed skull material for this taxon.

[5] Silvio Renesto (2005) found similarities in the postcranial skeleton of Pachystropheus and the thalattosaur genus Endennasaurus; according to Renesto, these similarities may indicate that Pachystropheus and Endennasaurus are close relatives, but they might as well simply be a case of a convergent evolution triggered by the aquatic lifestyle of both taxa.

[6] The placement as a thalattosaur has been supported by other researchers,[7] including a thorough analysis published in 2024, which found it to be a thalattosaur based on postcranial characters, including the presence of a "finely striated blade of the ilium", and a member of the subclade Askeptosauroidea based on its dumbbell-shaped radius.

The ribs of the body display pachyostosis and the vertebrae exhibit thickening, likely as an adaptation to living in a marine environment.