This study examined the effects of intergroup social justice on intergroup aggression inflicted against a wrongdoer by someone who was not directly involved in the wrongdoing (third party aggression). Specifically, it focused on whether third party aggression is retaliatory aggression or not. Sixty-four participants equally or unequally received lottery tickets from a fellow ingroup member; they then observed that the fellow ingroup member or an outgroup member unequally received such tickets from another outgroup member. After this observation, participants were given the opportunity to select the level of unpleasant noise that would be experienced by the outgroup member responsible for the unequal distribution. The results suggested that intragroup social justice enhanced group identification with the ingroup, which in turn enhanced perceived unfairness of intergroup distribution only when the victim was an ingroup member. That perceived unfairness then intensified hostility and aggressive behavior against the unfair outgroup member. Finally, the relationship between the psychological mechanisms of third party aggression and intergroup conflict in the real world was discussed.