To estimate the impact of tobacco-21 laws on smoking among young adults who are likely to smoke, and consider potential social multiplier effects. Quasi-experimental, observational study using new 2016–2017 survey data. United States 1,869 18–22 year-olds who have tried a combustible or electronic cigarette. Tobacco-21 laws raise the minimum legal sales age of cigarettes to 21 years. Logistic regressions compared the association between tobacco-21 laws and smoking among 18–20 year-olds with that for 21–22 year-olds. The older age group served as a comparison group that was not bound by these restrictions but could have been affected by correlated factors. Age-16 peer and parental tobacco use were considered as potential moderators. Self-reported recent smoking (past 30-day smoking) and current established smoking (recent smoking and lifetime consumption of at least 100 cigarettes). Exposure to tobacco-21 laws yielded a 39% reduction in the odds of both recent smoking (OR=0.61; 95% CI: 0.42, 0.89) and current established smoking (OR=0.61; 95% CI: 0.39, 0.97) among 18–20 year-olds who had ever tried cigarettes. This association exceeded the policy’s relationship with smoking among 21–22 year-olds. For current established smoking, the tobacco-21 reduction was amplified among those whose closest friends at age-16 used cigarettes (OR=0.50; 95% CI: 0.29, 0.87), consistent with peer effects moderating the policy’s impact on young adult smoking. Tobacco-21 laws appear to reduce smoking among 18–20 year-olds who have ever tried cigarettes.