Few studies have investigated the complex interactions among the individual- and community-level social risk factors that underlie adolescents' smoking behaviors. This study investigated whether community-level adult daily smoking prevalence is associated with adolescents' smoking and whether it moderates the associations between perceived friends' smoking approval and smoking behavior and adolescents' own smoking. Self-reported data from 1,190 youths (50.3% female; 13-18 years old) in 50 midsized Californian cities were obtained through telephone interviews. Community characteristics were obtained from 2010 GeoLytics data. Community adult daily smoking prevalence was ascertained from telephone interviews with 8,918 adults conducted in the same 50 cities. Multilevel analyses, controlling for individual and city characteristics, were used to predict adolescents' past 12-month smoking from perceived friends' smoking approval and smoking behavior and from community adult daily smoking prevalence. Results showed that perceived friends' smoking approval and behavior were associated positively with adolescents' smoking, as was the community-level prevalence of adult daily smoking. Furthermore, the association between perceived friends' smoking behavior and adolescents' own smoking was moderated by the prevalence of adult daily smokers in the community. Specifically, the association was stronger in cities with higher prevalence of adult smokers. These results suggest that adult community norms that are more supportive of smoking may enhance the influence of friends' smoking behavior. Therefore, interventions designed to prevent or reduce youths' smoking should also focus on reducing smoking by adults.