Several everyday examples imply that humans reciprocate not only towards direct perpetrators, but also to entire groups, thereby potentially affecting innocent bystanders. We test the hypothesis that people are predisposed to reciprocate against groups. In a laboratory experiment, subjects who were helped or harmed by another player’s action reacted by helping or harming another member of that player’s group. This group reciprocity was only observed when one group was seen as unfairly advantaged. Thus, activation of group reciprocity may be a causal mechanism that links perceived injustice to intergroup conflict. We discuss the relevance of group reciprocity to political and economic phenomena including violence, discrimination and team competition.