Atrial fibrillation (AF) has been associated with worse outcomes after primary percutaneous coronary intervention (PCI) for acute myocardial infarction. The aim of this study was to evaluate the incidence and impact of new-onset AF after primary PCI in patients with ST-segment elevation myocardial infarctions from the Harmonizing Outcomes With Revascularization and Stents in Acute Myocardial Infarction (HORIZONS-AMI) trial. HORIZONS-AMI was a large-scale, multicenter, international, randomized trial comparing different antithrombotic regimens and stents during primary PCI in patients with ST-segment elevation myocardial infarctions. Three-year ischemic and bleeding end points were compared between patients with and without new-onset AF after PCI. Of the 3,602 patients included in the HORIZONS-AMI study, 3,281 (91.1%) with sinus rhythm at initial presentation had primary PCI as their primary management strategy. Of these, new-onset AF developed in 147 (4.5%). Compared with patients without AF after PCI, patients with new-onset AF had higher 3-year rates of net adverse clinical events (46.5% vs 25.7%, p <0.0001), mortality (11.9% vs 6.3%, p = 0.01), reinfarction (16.4% vs 7.0%, p <0.0001), stroke (5.8% vs 1.5%, p <0.0001), and major bleeding (20.9% vs 8.2%, p <0.0001). By multivariate analysis, new-onset AF after PCI was a powerful independent predictor of net adverse clinical events (hazard ratio 1.74, 95% confidence interval 1.30 to 2.34, p = 0.0002) and major adverse cardiac events (hazard ratio 1.73, 95% confidence interval 1.27 to 2.36) at 3 years. In conclusion, new-onset AF after PCI for ST-segment elevation myocardial infarction was associated with markedly higher rates of adverse events and mortality.