In an effort to reduce exposure to secondhand smoke, many governments have enacted smoke-free laws in public spaces. To the extent that smoke-free laws change perceptions, norms and attitudes about tobacco use, these laws may also reduce the prevalence and intensity of cigarette consumption. Using nationally representative data on over four millions individuals spanning 15 years, we study the average effects of comprehensive, state-level indoor smoking bans on smoking prevalence in the United States. Our research design exploits within state variation in smoking prevalence, as well as the rapid diffusion of comprehensive bans across states between 2002 and 2010. Our results suggest that a comprehensive indoor smoking ban is associated with a 2.35% to 3.29% average reduction in smoking prevalence, a range that is robust to various specifications. These effects are roughly equivalent to a $1 to $1.50 increase in a state’s cigarette tax. Finally, we demonstrate that state-level indoor bans in bars may significantly decrease smoking prevalence in specific subpopulations, particularly young, female, low-income, and binge drinking individuals.