In real-world intergroup conflict, not all in-group members are equally threatened by the out-group. Yet, the impact of intragroup payoff asymmetry on the inclination to mutually cooperate during intergroup conflict and therefore to protect against out-group attacks, i.e., the “conflict-cooperation effect,” has not been investigated so far. In Study 1, we replicate previous research by using a novel experimental game paradigm, showing increased intragroup cooperation in the presence (vs. absence) of out-group threat under intragroup payoff symmetry. In Study 2, we find a conflict-cooperation effect among group members who are threatened (victims) as well as among group members who are not threatened (helpers) by the out-group. Intragroup cohesiveness, i.e., perceived closeness among in-group members, mediates the conflict-cooperation effect, particularly among selfishly-oriented individuals. Our results support the notion that intergroup conflict may have favored the evolutionary adaption of intragroup cooperation even when the in-group members were asymmetrically threatened by intergroup conflict.