Trophic subsidy is an ecological process consisting in the transference of nutrients between adjacent habitats, allowing the occupancy of less productive habitats by several species. We tested for the existence of trophic subsidy provided by aquatic plants of temporary ponds in maintaining individuals of the sand dune lizard Liolaemus occipitalis in southern Brazil. This lizard is a sit-and-wait predator restricted to coastal sand dunes. As these sand dunes occur in a mosaic of temporary ponds, we hypothesized the existence of a nutritional/energetic dependence of lizards from adjacent aquatic habitats. Our evaluation was based on carbon and nitrogen stable isotope analysis in association with gut content analysis of lizards sampled along an entire inundation cycle. Stomach contents revealed no seasonal variation in consumed prey item composition. Nevertheless, we detected changes in prey group composition. The volumetric contribution of Diptera increased over inundation classes (increase = 5%; peak = 61%). Carbon and nitrogen stable isotope ratios indicated that the L. occipitalis trophic web is mainly maintained by terrestrial sources. Isotope ratios also indicated the existence of aquatic contributions to tissue formation, but, despite its tendency to increase over the inundation process (10.7–13.3%), this trend lacks statistical support. We conclude that the studied population of L. occipitalis partially depends on trophic subsidy from aquatic sources and that any conservation plan for this endangered lizard species should take into account the preservation of non-habitats like temporary ponds.