Objectives: The authors aimed to develop a moral intervention and to determine whether it was more effective in preventing doping than an educational (i.e., knowledge-based) intervention; their primary outcome was doping likelihood, and the secondary outcomes were moral identity, moral disengagement, moral atmosphere, and anticipated guilt. Methods: Eligible athletes (N = 303) in the United Kingdom and Greece took part in the study. The authors randomly assigned 33 clubs to either the moral or the educational intervention. They measured outcomes pre- and postintervention and at 3- and 6-month follow-up. Results: Athletes in both interventions in both countries reported lower doping likelihood and moral disengagement and higher guilt from pre- to postintervention. These effects were maintained at the 3- and 6-month follow-ups. There were no effects on moral identity or moral atmosphere. Conclusions: In addition to disseminating information about doping, doping prevention programs should include content that focuses on moral variables.