Although cyberbullying is characterized by worrying prevalence rates and associated with a broad range of detrimental consequences, there is a lack of scientifically based and evaluated preventive strategies. Therefore, the present study introduces a theory-based cyberbullying prevention program (Media Heroes; German original: Medienhelden) and evaluates its effectiveness. In a pretest-posttest design (9-month interval), schools were asked to randomly assign their participating classes to either control or intervention group. Longitudinal data were available from 593 middle school students (M Age = 13.3 years, 53 % girls) out of 35 classes, who provided information on cyberbullying behavior as well as socio-demographic and psychosocial variables. While the present results revealed worrying prevalence rates of cyberbullying in middle school, multilevel analyses clearly demonstrate the program's effectiveness in reducing cyberbullying behavior within intervention classes in contrast to classes of the control group. Hence, this study presents a promising program which evidentially prevents cyberbullying in schools.