This study explored if moral norms as applied to the theory of planned behavior (TPB) account for additional variance in predicting intention to consume alcohol, smoke cigarettes, and engage in sexual activity among youth at a high school in Mexico. Additionally, it investigated if moral norms provide a moderating influence on the constructs of the theory: attitude, subjective norm, and perceived behavioral control for prediction of risky behavior intention. Multiple regression analyses identified predictive power of constructs; interactions of moral norms with the theory constructs were studied. Moral norms only significantly predicted sexual activity. Significant interactions were found between moral norms and the theory constructs for the three behaviors. Interventions aimed at preventing risky conduct among youth would benefit from strategies targeting beliefs in the moral order, especially because of its interaction with the other theoretical mechanisms.