Aims To compare the effectiveness and safety of clopidogrel, ticagrelor, and prasugrel in patients with ST-segment elevation myocardial infarction (STEMI) undergoing percutaneous coronary intervention (PCI). Methods and results Nationwide, registry-based study of STEMI patients treated with primary PCI (2011-17) and subsequently with aspirin and a P2Y(12) inhibitor. The effectiveness outcome was major adverse cardiovascular events (MACE) defined as a composite of recurrent myocardial infarction, repeat revascularization, stroke, or cardiovascular death at 12 months. The safety outcome was bleeding requiring hospitalization at 12 months. Multivariable logistic regression with average treatment effect modeling was used to calculate absolute and relative risks for outcomes standardized to the distributions of demographic characteristics of all included subjects. We included 10 832 patients; 1 697 were treated with clopidogrel, 7 508 with ticagrelor, and 1,627 with prasugrel. Median ages were 66, 63, and 59 years (P < 0.001). Standardized relative risks of MACE were 0.75 for ticagrelor vs. clopidogrel (95% confidence interval [CI], 0.64-0.83), 0.84 for prasugrel vs. clopidogrel (95% CI, 0.73-0.94), and 1.12 for prasugrel vs. ticagrelor (95% CI, 1.00-1.24). Standardized relative risks of bleeding were 0.77 for ticagrelor vs. clopidogrel (95% CI, 0.59-0.93), 0.89 for prasugrel vs. clopidogrel (95% CI, 0.64-1.15), and 1.17 for prasugrel vs. ticagrelor (95% CI, 0.89-1.45). Conclusion Ticagrelor and prasugrel were associated with lower risks of MACE after STEMI than clopidogrel, and ticagrelor was associated with a marginal reduction compared with prasugrel. The risk of bleeding was lower with ticagrelor compared with clopidogrel, but did not significantly differ between ticagrelor and prasugrel.