IMPORTANCE: Data on P2Y12 inhibitor monotherapy after short-duration dual antiplatelet therapy (DAPT) in patients undergoing percutaneous coronary intervention are limited. OBJECTIVE: To determine whether P2Y12 inhibitor monotherapy after 3 months of DAPT is noninferior to 12 months of DAPT in patients undergoing PCI. DESIGN, SETTING, AND PARTICIPANTS: The SMART-CHOICE trial was an open-label, noninferiority, randomized study that was conducted in 33 hospitals in Korea and included 2993 patients undergoing PCI with drug-eluting stents. Enrollment began March 18, 2014, and follow-up was completed July 19, 2018. INTERVENTIONS: Patients were randomly assigned to receive aspirin plus a P2Y12 inhibitor for 3 months and thereafter P2Y12 inhibitor alone (n = 1495) or DAPT for 12 months (n = 1498). MAIN OUTCOMES AND MEASURES: The primary end point was major adverse cardiac and cerebrovascular events (a composite of all-cause death, myocardial infarction, or stroke) at 12 months after the index procedure. Secondary end points included the components of the primary end point and bleeding defined as Bleeding Academic Research Consortium type 2 to 5. The noninferiority margin was 1.8%. RESULTS: Among 2993 patients who were randomized (mean age, 64 years; 795 women 26.6%), 2912 (97.3%) completed the trial. Adherence to the study protocol was 79.3% of the P2Y12 inhibitor monotherapy group and 95.2% of the DAPT group. At 12 months, major adverse cardiac and cerebrovascular events occurred in 42 patients in the P2Y12 inhibitor monotherapy group and in 36 patients in the DAPT group (2.9% vs 2.5%; difference, 0.4% 1-sided 95% CI, –∞% to 1.3%; P = .007 for noninferiority). There were no significant differences in all-cause death (21 1.4% vs 18 1.2%; hazard ratio HR, 1.18; 95% CI, 0.63-2.21; P = .61), myocardial infarction (11 0.8% vs 17 1.2%; HR, 0.66; 95% CI, 0.31-1.40; P = .28), or stroke (11 0.8% vs 5 0.3%; HR, 2.23; 95% CI, 0.78-6.43; P = .14) between the 2 groups. The rate of bleeding was significantly lower in the P2Y12 inhibitor monotherapy group than in the DAPT group (2.0% vs 3.4%; HR, 0.58; 95% CI, 0.36-0.92; P = .02). CONCLUSIONS AND RELEVANCE: Among patients undergoing percutaneous coronary intervention, P2Y12 inhibitor monotherapy after 3 months of DAPT compared with prolonged DAPT resulted in noninferior rates of major adverse cardiac and cerebrovascular events. Because of limitations in the study population and adherence, further research is needed in other populations. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT02079194