Current reviews revealed that there is a lack of effective programs and valuable effectiveness studies related to prevention of aggressive behavior and fostering of social competence in early adolescents participating in organized team sports (e.g., ball sports, such as soccer). Using a randomized controlled design, the present pilot study presents first results regarding the effectiveness of the preventive intervention program “Fairplayer.Sport” that was implemented with preadolescent soccer players (N = 145 preadolescents; aged 9–14 years; mean = 12.2 years) in organized team sport (13 soccer teams). Results revealed a reduction of aggressive behavior in the intervention groups compared to waiting‐control groups (small effect size). This effect remained stable 3 months after program implementation. Implications for planning and implementing preventive intervention programs are discussed.
