Despite the frequency and adverse effects of conflict, randomized controlled studies on interventions that could promote conflict resolution (e.g., among romantic couples) are scarce. One understudied intervention technique is mediation, which is a negotiation facilitated by a neutral third party. To test the impact of a mediator on couple conflict, we conducted a randomized controlled study involving 38 romantic couples who discussed a topic of recurrent disagreement either in the presence of a mediator or by means of a direct negotiation. The results show that romantic couples in the mediation condition, compared with those in direct negotiation, had a probability of reaching an agreement 1.39 times higher and reported higher satisfaction regarding the content and process of their discussions. In addition, the synchronicity of the couple’s skin conductance, a measure of arousal, correlated with the couple’s closeness and with the quality of their relationship. Our findings suggest that interventions based on mediation can have a beneficial impact on conflict resolution.