Diversification of threespine stickleback trophic morphology in relation to habitat has been studied extensively, but dentition has been ignored. Using samples of threespine stickleback from a range of habitats in the Pacific and Atlantic Basins, we characterize the morphology of individual teeth and their arrangement on the premaxilla. Although tooth morphology does not vary conspicuously, we observed striking differences in tooth arrangement between Pacific and Atlantic populations, significant variation among populations within basins, and pronounced sexual dimorphism in tooth number and arrangement within all populations from both basins.