Objective: An 18-site randomized trial was conducted to determine the effectiveness of social norms marketing (SNM) campaigns in reducing college student drinking. The SNM campaigns are intended to correct misperceptions of subjective drinking norms and thereby drive down alcohol consumption. Method: Institutions of higher education were randomly assigned to treatment and control groups. At the treatment group institutions, SNM campaigns delivered school-specific, data-driven messages through a mix of campus media venues. Cross-sectional student surveys were conducted by mail at baseline (n = 2,771) and at posttest 3 years later (n = 2,939). Hierarchical linear modeling was applied to examine multiple drinking outcomes, taking intraclass correlation into account. Results: Controlling for other predictors, having an SNM campaign was significantly associated with lower perceptions of student drinking levels and lower alcohol consumption, as measured by a composite drinking scale, recent maximum consumption, blood alcohol concentration for recent maximum consumption, drinks consumed when partying, and drinks consumed per week. A moderate mediating effect of normative perceptions on student drinking was demonstrated by an attenuation of the Experimental Group × Time interaction, ranging from 16.4% to 39.5% across measures. Additional models that took into account the intensity of SNM campaign activity at the treatment institutions suggested that there was a dose-response relationship. Conclusions: This study is the most rigorous evaluation of SNM campaigns conducted to date. Analysis revealed that students attending institutions that implemented an SNM campaign had a lower relative risk of alcohol consumption than students attending control group institutions.