Importance: Ischemic postconditioning of the heart during primary percutaneous coronary intervention (PCI) induced by repetitive interruptions of blood flow to the ischemic myocardial region immediately after reopening of the infarct-related arterymay limit myocardial damage. Objective: To determine whether ischemic postconditioning can improve the clinical outcomes in patients with ST-segment elevationmyocardial infarction (STEMI). Design, Setting, and Participants: In this multicenter, randomized clinical trial, patients with onset of symptoms within 12 hours, STEMI, and thrombolysis inmyocardial infarction (TIMI) grade 0-1 flow in the infarct-related artery at arrival were randomized to conventional PCI or postconditioning. Inclusion began on March 21, 2011, through February 2, 2014, and follow-up was completed on February 2, 2016. Analysis was based on intention to treat. Interventions: Patients were randomly allocated 1:1 to conventional primary PCI, including stent implantation, or postconditioning performed as 4 repeated 30-second balloon occlusions followed by 30 seconds of reperfusion immediately after opening of the infarct-related artery and before stent implantation. Main Outcome and Measures: A combination of all-cause death and hospitalization for heart failure. Results: During the inclusion period, 1234 patients (975 men [79.0%] and 259 women [21.0%]; mean [SD] age, 62  years) underwent randomization in the trial. Median follow-up was 38 months (interquartile range, 24-58 months). The primary outcome occurred in 69 patients (11.2%) who underwent conventional primary PCI and in 65 (10.5%) who underwent postconditioning (hazard ratio, 0.93; 95%CI, 0.66-1.30; P = .66). The hazard ratios were 0.75 (95%CI, 0.49-1.14; P = .18) for all-cause death and 0.99 (95%CI, 0.60-1.64; P = .96) for heart failure. Conclusions and Relevance: Routine ischemic postconditioning during primary PCI failed to reduce the composite outcome of death from any cause and hospitalization for heart failure in patients with STEMI and TIMI grade 0-1 flow at arrival.