We explored the relationships among locus of control, moral disengagement in sport, and rule transgression of athletes and whether or not moral disengagement mediates the relationship between locus of control and rule transgression. The participants were 462 athletes at a college in Taiwan. The instruments were the Sport Locus of Control Scale, the Moral Disengagement in Sport Scale, and the Rule Transgression Scale. The results revealed that a high external locus of control was positively related to moral disengagement in sport and frequent rule transgression. The relationship between locus of control and rule transgression was mediated by moral disengagement in sport. We suggest that athletes with an external locus of control should receive moral education to the end of enhancing the quality of sports contests and competitions through fewer rule transgressions and greater moral engagement.