Objective: Acts of relational aggression cause significant social and personal costs, and interventions are needed to reduce relational aggression in community as well as clinical settings. The present study used a persuasive message coupled with a self-affirmation manipulation to reduce relational aggression among a group of adolescents recruited from the community. Method: Participants (N = 503) all received a persuasive message designed to reduce relational aggression and were randomly allocated to participate in a self-affirming or nonaffirming task. Results: Findings demonstrated a significant reduction in relational aggression over 1-month among participants who were randomized to the self-affirmation condition (d = −0.50) in contrast with a small increase in relational aggression in the control condition (d = +0.20). Contrary to expectations, these effects were not mediated by message processing or changes in interpersonal affect. Conclusion: The present study used the novel approach of asking pupils to self-affirm following a persuasive message and showed that it was possible to reduce relational aggression. Self-affirmation shows considerable promise as a means of augmenting the delivery of interventions to reduce antisocial behavior in addition to other social and health behaviors.