In Puerto Rico, the number of nonnative Green Iguana, Iguana iguana, has increased and the species has proliferated throughout the island. Reports on diet in the iguana's native range indicate exclusive herbivory, but observations in their nonnative range occasionally include animal materials. The aim of our study was to determine the diet and trophic level of I. iguana in Puerto Rico using gut content and stable isotopic analysis of muscle tissue (tongue and leg). We found significant differences in the isotopic signature between leg and tongue tissue, which may be related to differing strategies for allocating nutrients during muscle formation. The isotopic analysis of δ¹⁵N and δ¹³C showed little enrichment of both muscle tissues from that of their diet, demonstrating that I. iguana is primarily an herbivore. However, gut contents provided evidence for a first report of I. iguana eating crabs (Uca spp.). The gut contents consisted primarily of black mangrove leaves (Avicennia germinans), suggesting a higher impact of herbivory on this species of mangrove. Another plant species of interest found in the gut was Brazilian pepper, Schinus terebinthifolius. We suspect that I. iguana may be a disperser of this aggressive invasive plant in Puerto Rico. Our study indicates that I. iguana impacts the native flora and fauna in Puerto Rico, and that the ecological role of this species in introduced ranges warrants further investigation.