Intergroup trust is vital for cooperation and societal well-being, but is harder to establish than interpersonal trust. We investigate whether expressions of negative emotions, in particular regret, following economic decisions can shape intergroup trust. In each of three studies participants were members of a group playing a two-round trust game with another group. In the first round, they observed an outgroup member who acted fairly or unfairly towards the ingroup and then expressed positive (i.e., happiness) or negative (i.e., regret, unhappiness) emotions about this behavior. In the second round, participants played with another outgroup member. Emotions displayed by the outgroup representative following unfair behavior in round 1 influenced participants' allocations in round 2, which were higher following regret and unhappiness than following positive emotions. Thus, emotions expressed by one outgroup member affected interactions with other members who had not communicated emotions. Findings of Study 3 revealed that these effects were driven by regret increasing intergroup trust, rather than by happiness decreasing it. Moreover, participants' allocations were predicted by their perceptions of the extent to which the outgroup representative wished to change her behavior. Together, the findings reveal that regret expressions influence intergroup trust by attenuating the detrimental effects of unfair behavior.