The evolutionary divergence of sexual signals is often important during the formation of new animal species, but our understanding of the origin of signal diversity is limited [1, 2]. Sensory drive, the optimization of communication signal efficiency through matching to the local environment, has been highlighted as a potential promoter of diversification and speciation [3]. The swordtail characin (Corynopoma riisei) is a tropical fish in which males display a flag-like ornament that elicits female foraging behavior during courtship. We show that the shape of the male ornament covaries with female diet across natural populations. More specifically, natural populations in which the female diet is more dominated by ants exhibit male ornaments more similar to the shape of an ant. Feeding experiments confirm that females habituated to a diet of ants prefer to bite at male ornaments from populations with a diet more dominated by ants. Our results show that the male ornament functions as a “fishing lure” that is diversifying in shape to match local variation in female search images employed during foraging. This direct link between variation in female feeding ecology and the evolutionary diversification of male sexual ornaments suggests that sensory drive may be a common engine of signal divergence.