We describe a new species of lizard of the genus Abronia from the Sierra Madre del Sur of Guerrero, Mexico. This species previously was confused with its presumed sister taxon Abronia deppii. The discovery to the north of the Balsas Basin of additional specimens referrable to A. deppii and a morphological comparison of these specimens to the populations of Abronia inhabiting the highlands of Guerrero to the south of the Balsas Basin clearly indicate that distinct species are present on each side of the Balsas Depression. Examination of the type material of A. deppii reveals that this name should be associated with the northern populations, and the southern population, often allocated to A. deppii, is an undescribed species. This new species is distinguished from A. deppii by having enlarged knob-like posterior head scales, no azygous scale between interparietal and interoccipital, a greater number of scale whorls on unregenerated tail, a greater number of infralabial scales, and dorsal and ventral coloration differences. We discuss the natural history of the new Guerrero species.