The studies described in this article explore the influence of an under-emphasized determinant of cooperative behavior in social dilemmas: affected third parties. Two experiments examined the effect of characteristics of third parties on individual cooperative behavior in social dilemmas, and identified mechanisms associated with these relationships. Study 1 demonstrated that third-party need affects individual cooperative behavior, and tested prosocial motivation and intra-group trust as mediators of this relationship. Study 2 provided further elaboration by demonstrating that individual cooperative behavior was reduced when a needy third party was low on reliability, and by showing that prosocial motivation and intra-group trust mediated this relationship. This research connects the literatures on individual cooperation in social dilemmas and relational job design, and demonstrates why third parties influence individuals' cooperative behavior.