Contrary to the tendency of mediators to defuse negative emotions between adversaries by treating them kindly, we demonstrate the surprising effectiveness of hostile mediators in resolving conflict. Hostile mediators generate greater willingness to reach agreements between adversaries (Experiment 1). Consequently, negotiators interacting with hostile mediators are better able to reach agreements in incentive-compatible negotiations than those interacting with nice mediators (Experiment 2). By serving as common enemies, hostile mediators cause adversaries in conflict to feel more connected and become more willing to reach agreement (Experiments 3 and 4). Finally, we manipulate the target of mediators’ hostility to document the moderating role of common enemies: mediators who directed their hostility toward both negotiators (bilateral hostility)—becoming a common enemy—increased willingness to reach agreement; those who directed hostility at just one negotiator (unilateral hostility) did not serve as common enemies, eliminating the hostile mediator effect (Experiment 5). We discuss theoretical and practical implications, and we suggest future directions. Data, as supplemental material, are available at http://dx.doi.org/10.1287/mnsc.2016.2431 . This paper was accepted by Yuval Rottenstreich, judgment and decision making.