An exceptionally well-preserved skull and associated dentary from the Chiquimil locality in the Andalhuala Formation (Huayquerian Age, late Miocene), Santa María Basin, Catamarca Province, northwestern Argentina, represent a new giant porcupine, Neosteiromys pattoni, sp. nov. N. pattoni is derived relative to Neosteiromys bombifrons Rovereto, 1914, the type species of the genus, by having pentalophodont m2–m3, a markedly infected angular region, more prominent masseteric crest, deeper masseteric fossa, and larger auditory bullae. Both species of Neosteiromys appear to have been adapted to eat harder, more abrasive food items than those consumed by extant porcupines, as inferred from their stronger masticatory apparatus. They may have inhabited more open environments than those of living Neotropical porcupines.