Introduction: Drinking alcohol has detrimental health consequences, and effective interventions to reduce hazardous drinking are needed. The self-regulation intervention of Mental Contrasting with Implementation Intentions (MCII) promotes behavior change across a variety of health behaviors. In this study, we tested if online delivery of MCII reduced hazardous drinking in people who were worried about their drinking. Method: Participants (N = 200, female = 107) were recruited online. They were randomized to learn MCII or solve simple math problems (control). Results: Immediately after the intervention, participants in the MCII condition (vs. control) reported an increased commitment to reduce drinking. After 1 month, they reported having taken action measured by the Readiness to Change drinking scale. When drinking was hazardous (Alcohol Use Disorders Identification Test = 8, n = 85), participants in the MCII condition indicated a decreased number of drinking days, exp([beta]) = 0.47, CI (confidence interval) [-1.322, -0.207], p = 0.02, and drinks per week, exp([beta]) = 0.57, CI [0.94, 5.514], p = 0.007, compared with the control condition. Discussion: These findings demonstrate that a brief, self-guided online intervention ("Mdn" = 28 minutes) can reduce drinking in people who worry about their drinking. Our findings show a higher impact in people at risk for hazardous drinking. Conclusion: MCII is scalable as an online intervention. Future studies should test the cost-effectiveness of the intervention in real-world settings.