This study describes the diet composition and prey selection of the Endangered Junín giant frog Telmatobius macrostomus, endemic to the central Andes of Peru. Prey items were recovered by forced regurgitation of stomach contents through gastric lavage. Top prey taxa in all samples (n = 9) consisted of a snail (Mollusca: Gastropoda: Hygrophila: Physidae; 78% frequency of occurrence) and an amphipod (Arthropoda: Malacostraca: Amphipoda: Hyalellidae; 56% frequency of occurrence). T. macrostomus appeared to select snails (family Physidae) and mayflies (family Baetidae) from the available prey in the environment. No vertebrate species were found in the stomach contents. Only 9 adults were found during this study (survey effort = 8.9 person-hours per frog), suggesting that adults of this species are rare and/or difficult to find. Although our sample size is limited, and the results need to be interpreted with caution, these findings provide important basic ecological data that can prove useful in the conservation of this species.