Diplacodon gigan is a new species of uintan brontothere recognized from an exquisitely preserved skull from the Wiggins Formation, Hot Springs County, Wyoming. Close affinity to Protitanotherium emarginatum has been suggested based on similarities of canine size and horn shape. However, evidence suggests that canine size and horn morphology were sexually dimorphic among brontotheres, and differences in these traits between single specimens or very small samples of specimens are suspect. Analysis of coefficients of variation of canines and other craniodental variables in brontotheres demonstrates a marked increase in the intraspecific variability of canines among horned brontotheres and closely related hornless species. The evolution of sexual dimorphism in the Brontotheriidae follows a pattern repeated in the evolutionary histories of other ungulates, including ruminants and rhinoceroses, where the evolution of sexually dimorphic canines or canine-like incisors commonly precedes the evolution of horns or horn-like cranial appendages. A phylogenetic analysis, which incorporates new character data on horn size, horn shape, canine size, and canine dimorphism, places D. gigan as the sister of D. elatus, and is the basis for the referral of this new species to Diplacodon. Phylogenetic analysis demonstrates that similarities between P. emarginatum and D. gigan, particularly with respect to canine and horn morphology, are plesiomorphic traits shared with basal horned brontotheres, such as Protitan. The shared nasal bone morphology of D. elatus and D. gigan, where the lateral margins of the nasal bone are angled markedly dorsalward, is a synapomorphy of the genus Diplacodon and is not observed in other brontotheres.