Liolaemus gracilis was cited in the categorization of the reptile habitat associations of Buenos Aires province in Argentina as the only Liolaemus lizard inhabiting the rocky grounds of the Tandilia System. We describe a new species of the genus Liolaemus which was previously confused with the widely ranging L. gracilis from the population of this mountain range. We examined samples of populations from coastal sand dunes and from the mountain range of Tandilia. Morphological characters and measurements were taken from specimens of both populations. We recorded ecological and life history notes of the saxicolous form in the field. The new species is an endemic and saxicolous form from Tandilia that can be distinguished from L. gracilis by indistinct or absent light dorsolateral stripes, small and segmental paravertebral spots, and by a throat with a denser, fine and diffuse spotted pattern. The ventral coloration is grayish plumb. Temporal scales are keeled or slightly keeled and the auricular scale is less differentiated. Morphometrically, the new species is larger in snout–vent length and head width but smaller in maximum body width and tail length than L. gracilis. The new species is oviparous and insectivorous.