We describe a new lagostomine rodent (Caviomorpha, Chinchillidae), Lagostomus telenkechanum, sp. nov., from the Arroyo Chasicó Formation (late Miocene; Buenos Aires Province, Argentina). Three characters permit its assignment to Lagostomus: suborbital canal for the infraorbital nerve limited by a bony lamina, well-developed posterior process of the palatine, and posterior maxillary foramen located at the midpoint of the M3. In addition, we identify the new species based on a unique character combination (e.g., small size, palatines reaching anterior to the level of the midpoint of P4, anterior margin of mesopterygoid fossa at the level of the anterior border of M3, upper cheek teeth more compressed anteroposteriorly and more obliquely implanted in the maxilla than in L. pretrichodactyla, L. compressidens, and L. maximus but less than in L. incisus, and thick cement layer in the hypoflexus). Because the early Miocene—middle Miocene genera, Prolagostomus and Pliolagostomus, are no longer recorded from the late Miocene, whereas the oldest record of the extant genus Lagostomus is recognized at that time, we consider this period as an important turnover time in the taxonomic composition of the Chinchillidae. This study increases understanding of the taxonomic diversity of the late Cenozoic lagostomines, contributing to the knowledge of the evolutionary and paleobiogeographic history of chinchillids.