Genital morphology reveals rapid diversification among species, and species-specific divergence in genital morphology may result in reproductive isolation and promote speciation. Natural selection against maladaptive hybridization may cause species-specific genital divergence. In this context, divergence in mating traits is expected to be greater between sympatric populations than between allopatric populations in a pair of species, known as reproductive character displacement (RCD). However, there are few examples of RCD in the genital morphology of closely related species. Additionally, processes leading to RCD have rarely been inferred. In this study, we examined RCD and its underlying mechanisms by focusing on species-specific genital morphologies of closely related Ohomopterusground beetle species. A morphological analysis showed patterns of RCD in species-specific genital parts in both sexes. Interspecific hybridization was confirmed by a mate choice experiment and by a population genetic analysis indicating extensive interspecific gene flow, suggesting that reinforcement is the most plausible process underlying the observed RCD. We found variation in the degree of displacement in contact zones, which may correspond with the ongoing process of genital evolution and speciation. Our results provide support for the lock-and-key hypothesis of genital evolution in closely related Ohomopterus species.