The postcranial skeleton of Mesosaurus tenuidens, a reptile from the Lower Permian of eastern South America and southern Africa, is redescribed and illustrated in detail. The number of presacral vertebrae varies in M. tenuidens from 29 to a maximum of 33, which falls just short of the maximum (34–35 presacrals) known in its close relative Stereosternum tumidum. A cleithrum is reported for the first time in a mesosaur. The head of the interclavicle is triangular, rather than diamond-shaped as in other basal reptiles, including S. tumidum. The carpus of M. tenuidens is apomorphic in that the intermedium and the lateral centrale are coossified. The lateral centrale pedis is absent. Reappraisal of the aquatic adaptations of M. tenuidens suggests strongly that this reptile was fully dedicated to an aquatic lifestyle. Pervasive pachyostosis of the postcrania probably allowed animals to maintain neutral buoyancy in the uppermost 3–4 m of the water column. Additionally, such pachyostosis may have helped to control rolling at the surface, or perhaps served to maintain forward momentum during periods of gliding while moving underwater. Limited movement at the elbow and the ankle suggests that M. tenuidens could not walk on land, but individuals may have been able to push themselves across terrestrial substrates, perhaps in a manner analogous to that seen in female extant marine turtles on nesting beaches.