Male–male competition over fertilization can select for harmful male genital structures that reduce the fitness of their mates, if the structures increase the male's fertilization success. During secondary contact between two allopatrically formed, closely related species, harmful male genitalia may also reduce the fitness of heterospecific females given interspecific copulation. We performed a laboratory experiment to determine whether the extent of genital spine exaggeration in Callosobruchus chinensis males affects the fitness of C. maculatus females by injuring their reproductive organs. We found that males with more exaggerated genital spines were more likely to injure the females via interspecific copulation and that the genital injury translated into fecundity loss. Thus, as predicted, reproductive interference by C. chinensis males on C. maculatus females is mediated by exaggeration of the genital spine, which is the evolutionary consequence of intraspecific male–male competition. Harmful male traits, such as genital spines, might generally affect the extent of interaction between closely related species.