Introduction. One of the most challenging issues in public health is preventing aggression and violent behavior, generally in the adolescent population. Intervention studies in this field, especially in Iran, were few. Moreover, their findings are controversial. Therefore, this study was conducted to investigate the effect of educational intervention based on the theory of planned behavior (TPB) on reducing aggression among male students. Method. This study used a randomized controlled trial design. The sample comprised 98 middle school students aged between 13 and 16 years (
14.28
±
0.7
). Educational intervention for the experimental group consisted of five sessions of 45-60 minutes. Data were collected using two self-administered questionnaires to measure aggression and constructs of TPB. Data were analyzed using paired
t
-test, independent
t
-test, and chi-square test at a significance level of 0.05. Results. After the intervention, the experimental group showed a significant increase in all TPB constructs except the subjective norms, compared to the control group (
p
<
0.001
). After two months of intervention, the mean score of the aggression behaviors in students in the experimental group showed a remarkable improvement in the experimental group, while the control group showed no significant difference. Conclusion. The findings of this study showed that the theory-based educational intervention was effective on the improvement of aggressive behavior. To achieve a significant change in perceived mental norms, more training sessions are recommended, and emphasis is placed on educating parents, peers, and school staff.
