The skull of the traversodontid cynodont Exaeretodon riograndensis, sp. nov. from the Santa Maria Formation, Rio Grande do Sul State, Brazil is described. The excellent preservation of one specimen reveals new cranial information about the genus. The presence of prootic crests in the extensive lateral flange, anterior to the fenestra ovalis, is a noteworthy basicranial trait, unique to this new taxon among non-mammalian cynodonts. These crests are interpreted as related to the origin of the posterior pterygoid muscle representing the first evidence, among non-mammalian cynodonts to mammals transition, of the attachment of this muscle onto the prootic bone. The new species is most similar to Exaeretodon frenguellii from the Ischigualasto Formation of Argentina. Small differences in the posterior projections of the jugal onto the squamosal and in the shape of prefrontals distinguish the two species. Additionally, the number of postcanines seems to be ontogenetically less variable in the Brazilian species. The postcanine number decreases from six plus one erupting in a small individual (preorbital length: 82.9 mm) to five plus one erupting in the largest specimen (pl. 121.5 mm).