Collective action is an effective way to maintain and realize social equity and justice. Social-psychological theory and research typically defines collective action as any action performed by group members to improve the status of a whole group in the context of collective disadvantage. As such, this literature typically focuses on disadvantaged groups’ self-regard collective action. Far less attention has been given to collective action in contexts characterized by protesting to promote the interests and status of out-group members. We propose a novel way to integrate the insights from social-psychological theories of collective action and intergroup contact theory to better understand what s those who have nothing to do with the social issues to help disadvantaged groups fight for their rights and interests. Three experiments were conducted to explore the influence of positive intergroup contact on altruistic collective action and the inner psychological process. Experiment 1 used the imaginative intergroup contact paradigm to manipulate positive intergroup contact to investigate the causal relationship between positive intergroup contact and altruistic collective action. 95 students were recruited and randomly assigned to the 2 conditions. It was found that positive intergroup contact promoted altruistic collective action, and system justification and the warmth dimension of intergroup evaluation mediated the influence of positive intergroup contact on altruistic collective action. The specific performance was that positive contact inhibited system justification, and then the bystander group members were more willing to participate in collective action to help the disadvantaged group. Besides, positive contact made the bystanders evaluate disadvantaged group members more positively (more warmth), and thus more willing to participate in altruistic collective action. Experiment 2 manipulated system justification to investigate the causal relationship. 75 students were recruited, and the findings showed that increased system justification inhibited collective action, because one should be unlikely to act against a system that one perceives as just. Experiment 3 manipulated warmth evaluation (69 students were recruited), and the finding showed that the more warmth the out-group (disadvantaged-group) members were described, the more willing the bystanders were to participate in altruistic collective action. When used as follow-ups to the original mediational test, Studies 2 and 3 provided the strongest evidence that system justification and perceived warmth of out-group members mediated the association between positive contact and altruistic collective action. In conclusion, the current research explores the causal relationship between positive intergroup contact and altruistic collective action, including the causal impact of two proposed mediators—system justification and warmth evaluation—on collective action. Beyond traditional prejudice reduction and collective action approaches, intergroup contact can bridge social divides and unite people in the struggle for social justice. Positive contact can, as suggested in the present research, convince members of the bystanders to support movements against inequality. Similarly, positive contact between different minority groups could lead them to unite and form new coalitions in the pursuit of social justice. The present research offers a compelling case for the importance of intergroup contact for social change.