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Aim. Understanding the forces shaping biodiversity patterns, particularly for groups of organisms with key functional roles, will help predict ecosystem responses to environmental changes. Our aim was to evaluate the relative role of different drivers in shaping diversity patterns of vertebrate herbivores, a group of organisms exerting a strong trophic influence in terrestrial Arctic ecosystems. This biome, traditionally perceived as homogeneous and low in biodiversity, includes wide variation in biotic and physical conditions and is currently undergoing major environmental change. Location. Arctic (including High Arctic, Low Arctic and Subarctic) Methods. We compiled available data on vertebrate (bird and mammal) herbivore distribution at a pan-Arctic scale, and used eight variables that represent the most relevant hypotheses to explain patterns of species richness. We used range maps rasterized on a 100 x 100 km equal-area grid to analyse richness patterns of all vertebrate herbivore species combined, and birds and mammalian herbivores separately. Results. Overall, patterns of herbivore species richness in the Arctic were positively related to plant productivity (measured with Normalized Difference Vegetation Index) and to the species richness of predators. Greater species richness of herbivores was also linked to areas with higher mean annual temperature. Species richness of bird and mammalian herbivores were related to the distance from the coast, with highest bird richness in coastal areas and mammal richness peaking further inland. Main conclusions. Herbivore richness in the Arctic is most strongly linked to primary productivity and the species richness of predators. Our results suggest that biotic interactions, with either higher or lower trophic levels or both, can drive patterns of species richness at a biome-wide scale. Rapid ongoing environmental changes in the Arctic are likely to affect herbivore diversity through both impacts on primary productivity and changes in predator communities via range expansion of predators from lower latitudes., Birdlife International and NatureServe. I.C.B. was supported bya post-doctoral fellowship funded by the Icelandic ResearchFund (Rannsóknasjóður, grant no. 152468-051), E.M.S. and V.T.R. by the Fram Centre through the Climate-EcologicalObservatory for Arctic Tundra (COAT). Funding was also avail-able through the Natural Sciences and Engineering ResearchCouncil (Canada) to D.S.H, final submission