The study of biometric variables and the colour patterns of different plumage parts allow sex discrimination of Bonelli's eagles, both in the hand and at a distance. Between 1999 and 2012, 175 Bonelli's eagles were captured in Spain, Portugal and France and sexed on the basis of genetic analysis or by verification of reproductive behaviour. Biometric variables were measured and coloration patterns were assigned to four plumage parts (tail feathers, undertail coverts, leg feathers and throat). Females were larger than males, all variables analysed, with the exception of wing length, showing significant differences. The greatest differences were in body length, tarsus width, body mass and hindclaw length. The study of coloration revealed differences in plumage patterns between males and females. Males had paler underparts than females. Bonelli's eagles may be sexed reliably through biometric measurements and by assessing and identifying colour patterns. This is the first study quantifying sexual dimorphism of the endangered Bonelli's eagle, for which several research and conservation programmes are now being implemented.