Background In patients with ST-segment elevation myocardial infarction (STEMI) with multivessel coronary artery disease, the time at which complete revascularization of nonculprit lesions should be performed remains unknown. Methods We performed an international, open-label, randomized, noninferiority trial at 37 sites in Europe. Patients in a hemodynamically stable condition who had STEMI and multivessel coronary artery disease were randomly assigned to undergo immediate multivessel percutaneous coronary intervention (PCI; immediate group) or PCI of the culprit lesion followed by staged multivessel PCI of nonculprit lesions within 19 to 45 days after the index procedure (staged group). The primary end point was a composite of death from any cause, nonfatal myocardial infarction, stroke, unplanned ischemia-driven revascularization, or hospitalization for heart failure at 1 year after randomization. The percentages of patients with a primary or secondary end-point event are provided as Kaplan-Meier estimates at 6 months and at 1 year. Results We assigned 418 patients to undergo immediate multivessel PCI and 422 to undergo staged multivessel PCI. A primary end-point event occurred in 35 patients (8.5%) in the immediate group as compared with 68 patients (16.3%) in the staged group (risk ratio, 0.52; 95% confidence interval, 0.38 to 0.72; P<0.001 for noninferiority and P<0.001 for superiority). Nonfatal myocardial infarction and unplanned ischemia-driven revascularization occurred in 8 patients (2.0%) and 17 patients (4.1%), respectively, in the immediate group and in 22 patients (5.3%) and 39 patients (9.3%), respectively, in the staged group. The risk of death from any cause, the risk of stroke, and the risk of hospitalization for heart failure appeared to be similar in the two groups. A total of 104 patients in the immediate group and 145 patients in the staged group had a serious adverse event. Conclusions Among patients in hemodynamically stable condition with STEMI and multivessel coronary artery disease, immediate multivessel PCI was noninferior to staged multivessel PCI with respect to the risk of death from any cause, nonfatal myocardial infarction, stroke, unplanned ischemia-driven revascularization, or hospitalization for heart failure at 1 year. (Supported by Boston Scientific; MULTISTARS AMI ClinicalTrials.gov number, NCT03135275.)