Background: Smoking was among the top contributors to deaths and disability as the prevalence among male adults remains high, and that among male youth increases in Indonesia. While local studies have shown high visibility of outdoor tobacco advertising around schools, the country still has no outdoor tobacco advertising ban. Objective: To examine the association between youth smoking behavior and measures of outdoor tobacco advertising density and proximity in Indonesia. Methods: We combined two primary data sources, including outdoor tobacco advertising and smoking behavior among male youth in Semarang city. We randomly selected and interviewed 400 male students at 20 high schools in the city. In addition, we interviewed 492 male adults who lived near the schools for comparison. Results: We found significant associations between smoking use among youth (but not among adults) and measures of outdoor tobacco advertising density and proximity in Semarang city. Youth at schools with medium and high density of outdoor tobacco advertising were up to 2.16 times more likely to smoke, compared to those with low density. Similarly, youth at senior high schools with proximity to outdoor tobacco advertising were 2.8 times more likely to smoke. Also, young people at poorer-neighborhood schools with a higher density of and proximity to outdoor tobacco advertising were up to 5.16 times more likely to smoke. Conclusion: There were significant associations between smoking use among male youth (but not among male adults) and measures of outdoor tobacco advertising density and proximity in Indonesia. This highlights the need to introduce an outdoor tobacco advertising ban effectively, at least near schools.