Although they have been considered distinct genera for over a century, ontogenetic analyses reveal that Triceratops and “Torosaurus” actually represent growth stages of a single genus. Major changes in cranial morphology—including the opening of parietal fenestrae and the elongation of the squamosals—occur rapidly, very late in Triceratops ontogeny and result in the characteristic ‘Torosaurus’ morphology. This report presents the results of a 10-year field study of the dinosaurs of the Hell Creek Formation in Montana and is based on a collection of over 50 specimens of Triceratops, including over 30 skulls, which have been amassed in that time, in addition to specimens from numerous other North American museums. This large sample of individuals reveals the full ontogenetic spectrum of Triceratops. The synonymy of Triceratops and ‘Torosaurus’ contributes to an unfolding view of extremely reduced dinosaur diversity just before the end of the Mesozoic Era.