Jellyfish are arguably one of the most important predators in the sea, and understanding their role in ecosystem functioning is critical. Modelling seems the best approach to understand and forecast their role, but these efforts are seriously hampered by the scant knowledge of the detailed diet composition of most species. We sampled a population of the largest scyphomedusa (Chrysaora plocamia) from an upwelling centre in the Humboldt Current Upwelling System. Fish eggs/larvae and holoplanktonic crustaceans represented a substantial proportion (between 52.4% and 99.3%) of the diet of C. plocamia and that of other species within the genus Chrysaora, thus highlighting their potential impact in ecosystems sustaining large fisheries. The diet composition of C. plocamia displayed a strong temporal variability pattern that seemingly reflects the control of upwelling pulses on population dynamics of the species being predated by this medusa.