Background: Using data from the ODYSSEY OUTCOMES trial (NCT01663402), we sought to identify factors associated with the development of incident atrial fibrillation in patients with recent acute coronary syndrome without prior atrial fibrillation and to determine whether alirocumab treatment influenced risk of incident atrial fibrillation. Methods: ODYSSEY OUTCOMES compared alirocumab treatment with placebo in 18,924 patients with recent acute coronary syndrome and dyslipidemia despite high-intensity or maximum-tolerated statin therapy. The primary outcome of major adverse cardiovascular events (MACE) comprised death from coronary heart disease, non-fatal myocardial infarction, fatal or non-fatal ischemic stroke, or unstable angina requiring hospitalization. Patients were classified as having previous atrial fibrillation (present prior to or at randomization) or no previous atrial fibrillation. A multivariable model was used to determine factors associated with incident atrial fibrillation. Results: Among 18,262 participants without prior atrial fibrillation at baseline, 499 (2.7%) had incident atrial fibrillation during follow-up. Older age, history of heart failure or myocardial infarction, and higher body mass index were significantly associated with incident atrial fibrillation. Treatment with alirocumab or placebo did not influence the cumulative incidence of atrial fibrillation (hazard ratio 0.91; 95% confidence interval, 0.77-1.09). Patients with vs without a history of atrial fibrillation had a higher incidence of MACE (8.8 vs 3.7 events per 100 patient-years), without significant interaction between atrial fibrillation and randomized treatment on risk of MACE (Pinteraction = .78). Conclusions: While alirocumab did not modify risk of incident atrial fibrillation after acute coronary syndrome, it did reduce the risk of MACE, regardless of prior atrial fibrillation history. History of atrial fibrillation is an independent predictor of recurrent cardiovascular events after acute coronary syndrome.