Most lizard species feed on small arthropods, and although some are omnivorous, only a few species are strict herbivores. We studied the diet of Dicrodon guttulatum, a teiid lizard endemic to the arid coastal deserts and dry forests of northern Peru. Herbivory by this lizard has been identified as a potential limiting factor in the regeneration of plant communities in these dry ecosystems. We collected gastric and fecal samples of adult males, adult females, and juveniles of D. guttulatum in different plant communities. Based on the ontogenetic shift from insectivory to herbivory observed in other herbivorous lizards, we hypothesized that juvenile D. guttulatum would have an omnivorous or insectivorous diet. We found D. guttulatum to be almost exclusively herbivorous (78–100%) and to feed largely (12–95%) on mesquite (Prosopis pallida) leaves and flowers across all plant communities and locations. Contrary to our predictions, there was a large overlap in diet between adult males and juveniles, whereas adult females were most likely to include plant species other than P. pallida in their diet. The consistency of herbivory in both juveniles and adults makes the origin of herbivory in D. guttulatum very interesting. We discuss potential factors promoting the evolution of herbivory in this species and the importance of lizard herbivory in the dry ecosystems of northern Peru.