We describe a new species of Thomasomys from the Vilcabamba Cordillera, Cuzco, Peru. This thomasomyine is a medium-sized, small-eared, and long-tailed rodent similar in external and cranial features to Thomasomys silvestris, a species from the western Andean slopes of Ecuador. The new species presents a unique combination of characters that includes the absence of genal vibrissae, absence of a "gap" between the hypothenar and thenar pads, short incisive foramina, and a primitive pattern of carotid circulation. The proposed new species is known only from the type locality, suggesting that its restricted distribution could be attributed to the existence of a relict fauna in the area.