Theory indicates that ecologically-driven disruptive selection can have several outcomes, among them speciation and sexual dimorphism. It is expected that sexual dimorphism, which can evolve quickly, will remove the disruptive selection leading to adaptive speciation and therefore make both outcomes unlikely to occur together. We examined the magnitude of sexual dimorphism and species variation in shape and size of three-spine sticklebacks (Gasterosteus aculeatus) from two lakes in British Columbia that contain limnetic and benthic species pairs. Geometric morphometrics analyses of shape indicate that both sexual dimorphism and species differences are present, and that variation is even more extreme between the sexes than between species. To understand the conditions under which sexual dimorphism and speciation can occur, we performed numerical simulations with an ecological model that contains more than a single axis of variation. By considering disruptive selection on two axes of phenotypic variation, we propose that simultaneous sexual dimorphism and speciation are more likely than previously thought, and that these conditions may explain the variation seen in stickleback populations.