A survey of the postcranial anatomy of a specimen of Diademodon tetragonus recovered from the Upper Omingonde Formation in Namibia resulted in the recognition of diagnostic characters in the axis, scapula, interclavicle, manubrium, sternebrae, humerus, ilium, ischium, and femur. Our comparative analysis shows that these and other postcranial features distinguish Diademodon tetragonus from other cynognathians. The presence of ossified sternal elements (manubrium and sternebrae) in Diademodon tetragonus stands out because they are otherwise only present in tritylodontids among nonmammaliaform cynodonts. It is suggested that this feature is not linked to body size but could be phylogenetically informative. A review of the postcranial anatomy of specimens previously identified as Diademodon and ?Cynognathus/ ?Diademodon shows that only a few of them can be assigned to Diademodon tetragonus.