A relatively complete skull of Hypsosteiromys with a well-preserved natural cranial endocast was recovered from the Colhuehuapi Member, Sarmiento Formation (lower Miocene) at Gran Barranca, Chubut Province, Argentina. The only known skull of this genus, it differs from those of other erethizontids because it is continuously very low, the ascending ramus of the premaxillae extends behind the posterior margins of the nasals, and the external auditory meatus has a ventral furrow. Comparison of the endocast with the brains of extant Erethizontidae demonstrates remarkable neuromorphological homogeneity, despite differences in size. Erethizontid brains have rounded, telencephalic hemispheres with prominent frontal lobes and small olfactory bulbs, and lack temporal lobes and neocortical sulci. Thus, the Gran Barranca skull shows that the external neuromorphology of erethizontids has not changed substantially since at least the early Miocene.