This study presents the first accurate record of the toxodontid Toxodon platensis in the Late Pleistocene of the San Luis Province, central-western Argentina. The specimen, CHN-UNLC-PV 7, is a partial skull recovered from the Barranquita Formation at Río Cañada Honda locality. Stratigraphic and sedimentological analyses of the Río Cañada Honda section identify a mud-dominated sequence with interstratified sandy-gravelly facies, reflecting a depositional environment characterized by fluvial and alluvial-eolian conditions. These deposits fill depressed erosive surfaces on the igneous-metamorphic basement of the Sierra de San Luis, known locally as “pampas de altura”. The anatomical study of the skull reveals features consistent with Toxodon platensis , including its characteristic dental morphology and a relatively flattened cranial profile. Metrical comparisons indicate a smaller size than some contemporary specimens, suggesting a subadult ontogenetic stage. Furthermore, the use of medical CT scans allows for the description of poorly known intracranial anatomy of this taxon and the discussion of previous interpretations. The resulting braincast shows oval-shaped cerebral hemispheres with clear but shallow gyrification, well-developed temporal lobes, and protruding ‘plowshare-shaped’ olfactory lobes. The epitympanic sinus is tear-shaped and smaller than in other toxodontids. Paleobiogeographically, the occurrence of T. platensis in fluvial sediments of the Barranquita Formation serves as a critical geographic bridge within the “Arid Diagonal” in Argentina, filling a regional gap between the eastern lowland records and those of the Andean region.