Abstract Invasive species alter habitats and biological communities. The giant river prawn Macrobrachium rosenbergii (de Man 1879) was introduced to Brazil for aquaculture, and invasive populations have established in the Amazon Delta region where they are believed to pose a risk to the native aquatic fauna. To assess potential impacts, we performed dietary metabarcoding using generalist COI primers on 105 stomach contents collected from prawns from the southern Amazon Delta. Overall, M. rosenbergii presents an opportunistic and generalist diet that reflects the dominant aquatic and terrestrial fauna of the region, including the orders Diptera, Characiformes, and Lepidoptera as dominant dietary items in terms of both frequency of occurrence and richness. One unidentified congeneric species was identified in the diet, indicating the potential for negative effects on the native prawn fauna. Additionally, while there is a general overlap in the diet for all categories of sex and reproductive phase, smaller immature individuals and molted females showed reduced diversity in their diet, suggesting limitations in prey handling or access. We conclude that dietary metabarcoding of opportunistic generalists and/or detritivores appears to be a potentially useful tool for monitoring biodiversity as well as understanding their role in the food web.