The dietary composition of the western Derby eland (Taurotragus derbianus derbianus), a critically endangered antelope, was investigated using microhistological analyses of faeces. Samples were collected in the Niokolo Koba National Park, the refuge of the last wild population, and in the Bandia wildlife reserve, where the animals foraged on natural and supplementary food. Leaves, shoots of woody plants, and fruits were the three major components at both sites. They formed 98.8% of diet volume in the wild and 77.5% in the Bandia Reserve where supplementary food reduced the consumption of natural components but maintained the total content of major components together with supplement at 99.2%. Other components such as forbs and grasses appeared in low proportions, generally below 5% of the mean volume. Leaves of Boscia angustifolia, Grewia bicolor, Hymenocardia acida, and Ziziphus mauritiana, and fruits of Acacia spp. and Strychnos spinosa were identified as part of the diet in the wild. In the Bandia Reserve, the proportions of diet components did not differ between males and females, but there were differences in consumption of supplementary food between age classes. The results indicate that in the dry season the western Derby eland behaves as a pure browser, consuming grasses in negligible amounts. Consequently, woody savanna habitat is necessary for future conservation enclosures to ensure adequate natural forage resources for animals and thus to avoid food supplementing that presents a risk of dietary shifts in animals in captivity.