We describe the diet of a Liolaemus cuyanus population from the Monte of San Juan Province in Argentina. Diet composition, trophic niche breadth, age, sexual, and seasonal variation were analyzed. In addition, we investigated the relationship between L. cuyanus morphology and size of prey consumed. From the stomach contents of 105 lizards, we found that L. cuyanus is an omnivore, feeding mainly on ants, hemipterans, coleopterans (larvae and adults), and reproductive plant structures (seeds, fruits, and flowers) of the genera Lycium, Atriplex, Larrea, and Portulaca. We observed differences between juvenile and adult diets, probably associated with body- and head-size restrictions. Conversely, sexes did not differ in diet, in spite of head- and body-size sexual dimorphism. Seasonal variation in diet was observed in juveniles, whereas adults did not show substantial seasonal changes in the diet. Our findings show that L. cuyanus is opportunistic in diet and that diet appears to vary depending on age and morphology.