Inter- and intra-sexual selection are important driving forces that lead to diversification of sexual characteristics. Tetragnatha spiders have elongated chelicerae and sexual dimorphism in chelicera length whose magnitude varies among species. Because they use their chelicerae during copulation and as weapons in male–male competition, this divergence reflects repeated inter- and intra-sexual selection. To infer the causes of chelicera length diversity, we examined the roles of the elongated chelicerae in copulatory behaviour of a Tetragnatha species and clarified the interspecific sexual dimorphism of chelicera length in 15 species. The longer chelicerae of female Tetragnatha allowed females to kill males, suggesting female rejection of mating using the elongated chelicerae. Comparisons based on independent contrasts showed that chelicera length was significantly positively correlated between the sexes across species, possibly reflecting sexually antagonistic coevolution of male and female chelicera lengths. However, some species showed male-biased sexual dimorphism, indicating that intra-sexual selection (male competition) may be more important than inter-sexual competition in some species. Our results demonstrate that both inter-sexual and intra-sexual selection led to inter-specific variation in sexual dimorphism of chelicera length in this group.