--- - |2 The theory of reasoned action (ToRA) has been proposed as a framework for cyberbullying prevention design, targeting attitudes and norms. In this study effects of a long (10 weekly sessions) and a short (one day, four sessions) cyberbullying prevention program based on the ToRA were compared with a control group over 9 months. Longitudinal data from 722 students (mean age = 13.36) on cyberbullying, somatic symptoms, attitudes, and norms were analyzed within a structural equation model. Participation in the long intervention group significantly reduced cyberbullying (d = −0.584) and somatic symptoms (d = −0.316). No between‐group differences emerged for attitudes and norms. Developmental trajectories and associations were found to be as suggested by ToRA in both cross‐sectional and change‐score analyses. - Journal of Research on Adolescence, EarlyView.