Exaggerated traits can be the result of both sexual selection, including mate choice and male-male competition, and natural selection. Comparing the morphology of exaggerated traits between sexes, and observing interactions involving that trait, can be a powerful tool in understanding the evolution of exaggerated traits, mating systems, and life history. The leaf beetle Sagra femorata has enlarged hind legs, but little attention has been paid to this morphology and its function. We found sexual dimorphism in the hind legs of Sagra femorata and observed competitive male-male interactions that involved the hind legs. Males grasped and removed rival males with their hind legs, both to guard females while mating and to defend their feeding territories. Moreover, male hind legs were not used in courtship or to coercively grasp mates. These results indicate that the evolution of enlarged hind legs in leaf beetle Sagra femorata is likely the result of male-male competition.