Lagostomine rodents (Caviomorpha, Chinchillidae) are very abundant in late Cenozoic vertebrate associations of the Pampean area but the study of their systematics has been mostly limited to a handful of works from the late 19^th Century and early 20^th Century. Although seven species of Lagostomus were described for the Chapadmalal Formation (late Pliocene; Buenos Aires Province, Argentina), the holotypes of these species do not present precise stratigraphic provenance and have not been studied since their original description. This study of lagostomines from the Chapadmalal Formation with precise stratigraphic provenance gives way to the recognition of at least three species: Lagostomus incisus, L. compressidens and L. euplasius. The comparative study of the holotypes of the previously recognized species in this unit indicates that ‘Viscaccia indefinita’ is a junior synonym of L. compressidens while ‘V. loberiaense’, ‘V. definita’ and ‘V. chapalmalense’ are junior synonyms of L. euplasius. The validity of ‘Viscaccia arcuata’ and its inclusion in the Chapadmalalan fauna has to be re-evaluated. This work clarifies part of the systematics of the genus Lagostomus, thus contributing to the comprehension of the taxonomic diversity of chinchillids during their most recent evolutionary history.