Background: Limited evidence exists on effects of school-based nutrition policies. This study explored the influence of mandatory versus voluntary provincial school nutrition policies on student eating behaviors. Methods: A cross-sectional, school-based survey assessed student eating behaviors using self-report survey measures in a representative sample of Canadian high school students from 7 provinces (N = 12,110). Provincial school nutrition policies were characterized as mandatory or voluntary. Healthful and nonhealthful eating behaviors were analyzed across sociodemographic characteristics. Regression models were used to assess the association between policy type and eating behaviors, and to explore potential moderating variables. Results: Healthful and nonhealthful eating behaviors differed significantly across several sociodemographic characteristics. Overall, neither healthful nor nonhealthful eating behaviors differed significantly between schools with voluntary and mandatory nutrition policies (odds ratio [OR] = 0.83, 95% confidence interval [CI] 0.64-1.08; OR = 1.16, 95% CI 0.83-1.64). Frequency of buying lunch at school and buying lunch out moderated the association between policy enforcement level and healthful eating behaviors (p = 0.0472, p = 0.0119). Frequency of buying lunch out moderated the association between policy enforcement levels and nonhealthful eating behaviors (p = 0.0009). Conclusions: This study documents nonhealthful components of Canadian adolescents' diets, and the results highlight important areas for future research in assessing the effectiveness of school nutrition policies.