Theropod dinosaurs dominated Earth's terrestrial ecosystem as a diverse group of predators for over 160 million years, yet little is known about their foraging ecology. Ranging from the chicken-sized Microraptor up to the whale-sized Giganotosaurus, maintaining a balanced energy budget presented a major challenge in the face of intense competition and the demands of ontogenetic growth. Facultative scavenging, a behaviour present in almost all modern predators, may have been an important behaviour used to supplement energetically expensive lifestyles. By using agent-based models based on the allometric relationship between size and foraging behaviours, we show that theropods between 27 kg and 1044 kg would have gained a significant energetic advantage over individuals at both the small and large extremes of theropod body mass through their scavenging efficiency. These results were robust to rate of competition, primary productivity, and detection distance. Our models demonstrate the potential importance of facultative scavenging in theropods and the role of body size in defining its prevalence in Mesozoic terrestrial systems.