In sexually size-dimorphic species, physiological constraints derived from differences in body size may determine different food requirements and thus a trophic niche divergence between males and females. These relationships between sexual size dimorphism (SSD) and dietary overlap are not well understood in birds. We analyzed differences between the sexes in diet composition, dietary diversity, diet selection, and volume and density of droppings, as well as the dietary overlap between sexes, in the Great Bustard (Otis tarda), the species showing the highest SSD among birds. We discuss the differences that we found in relation to various predictions derived from ecological and physiological differences between the sexes, under the hypothesis that these differences are ultimately determined by the strong SSD of this species. As expected, our best linear mixed-effects food selection models included sex as a main factor explaining differences in diet composition and dietary diversity of Great Bustards throughout the annual cycle. Both sexes were mostly herbivorous, consuming legumes when they were available. Males consumed fewer arthropods, but of significantly larger size, than females. The droppings of males were larger, heavier, and slightly denser than those of females. Males showed higher dietary diversity than females, except during the postmating season. The mean dietary overlap between the sexes was 0.7, one of the smallest values among birds. Overall, our results suggest that the species' extreme SSD along with the distinct reproductive role of each sex might explain the trophic niche divergence in the Great Bustard.