Background: Media literacy programs have shown potential for reduction of adolescent tobacco use. We aimed to determine if an anti-smoking media literacy curriculum improves students' media literacy and affects factors related to adolescent smoking. Methods: We recruited 1170 9th-grade students from 64 classrooms in 3 public urban high schools. Students were randomized by classroom to a media literacy curriculum versus a standard educational program. In an intent-to-treat analysis, we used multilevel modeling to determine if changes in study outcomes were associated with the curricular intervention, controlling for baseline student covariates and the clustering of students within classrooms. Results: Among participants, mean age was 14.5 years and 51% were male, with no significant differences in baseline characteristics between groups. Smoking media literacy changed more among intervention participants compared with control participants (0.24 vs. 0.08, p < 0.001). Compared with controls, intervention students exhibited a greater reduction in the perceived prevalence of smoking (-14.0% vs. -4.6%, p < 0.001). Among those initially susceptible to smoking, intervention participants more commonly reverted to being nonsusceptible post-intervention (24% vs. 16%, p = 0.08). Conclusions: A school-based media literacy curriculum is more effective than a standard educational program in teaching media literacy and improving perceptions of the true prevalence of smoking among adolescents.