Pachystropheus rhaeticus is a primarily marine diapsid reptile found in Upper Triassic (Rhaetian) deposits in Britain and continental Europe. Although known for a long time, Pachystropheus remains an enigma. It was identified as the oldest choristodere based on similarities between postcranial bones and a sole cranial element. Here, we re-study all available Pachystropheus material, including new specimens, identifying all skeletal elements that are plausibly referrable to the genus. We reject some previous dubious identifications and show that reports of an ectopterygoid, the only cranial element of Pachystropheus described, were incorrect and that supposed skull elements belong to coelacanth fishes. Furthermore, we present CT data for the first time and new palynological data from Pachystropheus-bearing strata, providing a clearer picture of its stratigraphic distribution and the paleoecology of the Penarth Group. The reassessment of its phylogeny using new character informative data, places Pachystropheus as the last of the thalattosaurs, a lineage of marine reptiles abundant throughout the Triassic. We demonstrate that a specimen of Pachystropheus previously reported from the Lower Jurassic is more likely from the Late Rhaetian, and therefore the taxon does not extend across the end-Triassic Extinction. Moreover, this extends the known geological range of thalattosaurs from the Norian–Rhaetian boundary to the latest Rhaetian. Similarities in the postcranial skeleton between Pachystropheus and other askeptosauroid thalattosaurs, particularly Endennasaurus acutirostris, suggest it was able to move on land, but was most likely a primarily marine predator with an ecological niche distinct from coeval marine reptiles (placodonts, ichthyosaurs) and carnivorous fishes (hybodont sharks, actinopterygians).