The Black Caiman (Melanosuchus niger) experienced a dramatic population decline in the mid-20th century, becoming extinct or locally rare over most of its range due to habitat destruction and the commercial value of its hide. As the success and re-establishment of the species is now dependent on conservation efforts throughout the Amazon basin, Black Caimans require continuous monitoring despite extensive current legal protection. Although such efforts have mitigated the threat of human harvesting, a key issue facing the species today is ecological competition from sympatric Spectacled Caimans (Caiman crocodilus). In this study, we investigated this inferred competition by detailing the dietary overlap between Black Caimans and Spectacled Caimans in the Pacaya Samiria National Reserve in 2009. Using an adaptation of the hose-Heimlich technique, we collected and then compared the stomach contents of several individuals of each species. We found that although the proportions of prey items in their stomach contents varied seasonally, the dietary overlap between the two species remained high, suggesting intense competition for food resources. In addition to seasonal changes, the diet composition of Black Caimans also shifted ontogenetically. Young Black Caimans primarily ate insects and crustaceans, whereas larger individuals mostly preyed on fish, reducing intraspecific competition between crocodilian adults and young. Our findings suggest that Spectacled Caimans will continue to hinder the recovery of the Black Caiman population.