BackgroundPostoperative atrial fibrillation (POAF) is common after cardiac surgery, but little is known about its incidence and natural course after noncardiac surgery. We evaluated the natural course and clinical impact of POAF and the long‐term impact of anticoagulation therapy in patients without a history of atrial fibrillation (AF) undergoing noncardiac surgery.Methods and ResultsWe retrospectively analyzed the database of Asan Medical Center (Seoul, Korea) to identify patients who developed new‐onset POAF after undergoing noncardiac surgery between January 2006 and January 2016. The main outcomes were AF recurrence, thromboembolic event, and major bleeding during follow‐up. Of 322 688 patients who underwent noncardiac surgery, 315 patients (mean age, 66.4 years; 64.4% male) had new‐onset POAF with regular rhythm monitoring after discharge. AF recurred in 53 (16.8%) during 2 years of follow‐up. Hypertension (hazard ratio, 2.12;P=0.02), moderate‐to‐severe left atrial enlargement (hazard ratio, 2.33;P=0.007) were independently associated with recurrence. Patients with recurrent AF had higher risks of thromboembolic events (11.2% versus 0.8%;P<0.001) and major bleeding (26.9% versus 4.1%;P<0.001) than those without recurrence. Patients with recurrent AF and without anticoagulation were especially predisposed to thromboembolic events (P<0.001). Overall, anticoagulation therapy was not significantly associated with thromboembolic events (1.4% versus 2.5%,P=0.95).ConclusionsAF recurred in 16.8% of patients with POAF after noncardiac surgery. AF recurrence was associated with higher risks of adverse clinical outcomes. Considering the high risk of anticoagulation‐related bleeding, the benefits of routine anticoagulation should be carefully weighed in this population. Active surveillance for AF recurrence is warranted.