Background Aggression is often divided into reactive and proactive forms. Reactive aggression is typically thought to encompass ‘blaming others’ and ‘assuming the worst’, while proactive aggression relates to ‘self‐centeredness’ and ‘minimising/mislabelling’. Aim Our aim was to evaluate relationships between reactive and proactive aggression and cognitive distortions and to test whether changes in these cognitions relate to changes in aggression. Methods A total of 151 adolescents (60% boys; mean age 15.05 years, standard deviation 1.28) were enrolled in an evidence‐based intervention to reduce aggression. Due to attrition and anomalous responses, the post‐intervention sample involved 80 adolescents. Correlation and linear regression analyses were used to investigate the relationship between cognitive distortions and aggression. Results Blaming others was related to reactive aggression before the intervention, while all cognitive distortions were related to proactive aggression both pre‐ and post‐intervention. Changes in reactive aggression were uniquely predicted by blaming others, while changes in proactive aggression were predicted by changes in cognitive distortions overall. Conclusion To our knowledge, this study is the first to show a relationship between changes in cognitive distortions and changes in aggression. Treatment of reactive aggression may benefit from focusing primarily on reducing cognitive distortions involving misattribution of blame to others. Copyright © 2017 John Wiley & Sons, Ltd.