Studying the diet of wild animals is central for understanding their flexibility in food requirements. The Black Harrier Circus maurus is an endangered raptor in South Africa and Namibia. To date, information about the diet of the species is insufficient for a comprehensive understanding of its ecology. We studied the diet composition of breeding Black Harriers using c. 1 000 pellets (>1 700 identified prey) collected at nest sites in two geographical regions (coastal vs inland) over 10 breeding seasons (2006-2015). We show the importance of small mammals in Black Harrier diet (64.4% and 78.2% of prey and consumed biomass, respectively), with the four-striped mouse Rhabdomys pumilio being a main trophic resource. We also reveal the importance of birds and reptiles as alternative prey, particularly in inland regions, and show inter-annual variations in diet in both regions. Our study confirms that this species can be considered a small mammal specialist. Specialist predators are more vulnerable than generalist ones and diet specialisation has been linked with a poorer conservation status in other species. Our results thus have implications for the conservation of this species in southern Africa. These are highlighted for the long-term sustainability of this threatened endemic species.