Alcohol consumption in adolescence is associated with problem drinking in later life, and there is a need to develop evidence-based interventions to reduce adolescent alcohol consumption. The aims of the present study were to test the ability of a very brief intervention based on self-affirmation theory to reduce alcohol consumption in a sample of adolescents and to examine potential mediators of the effects.67 adolescents were randomly allocated either to form a self-affirming implementation intention or to complete a distractor task. All participants were exposed to a threatening message concerning the health risks of alcohol consumption. The main outcome measure was subsequent alcohol intake, but message processing (operationalized as perceived threat and message derogation), behavioral intention, and self-efficacy were also measured as potential mediators.The intervention produced a significant decrease in alcohol consumption: Participants in the self-affirming implementation intention condition consumed 2.48 fewer grams of pure alcohol per day at the end of the study than adolescents who completed the distractor task. The effect was not mediated by perceived threat, message derogation, behavioral intention, or self-efficacy.The findings provide support for the efficacy of the self-affirming implementation intention for promoting health behavior change and extend previous research by testing an adolescent sample and observing longer term effects. Further research is needed to find out what mediates the effects of self-affirming implementation intentions on health behavior change.