Importance: New-onset postoperative atrial fibrillation (POAF) is a common complication of cardiac surgery. However, data on the long-term risk of thromboembolism in patients who develop POAF after heart valve surgery are conflicting. In addition, data on stroke prophylaxis in this setting are lacking. Objective: To assess the long-term risk of thromboembolism in patients developing new-onset POAF after isolated left-sided heart valve surgery relative to patients with nonsurgical, nonvalvular atrial fibrillation (NVAF). Design, Setting, and Participants: This observational cohort study was conducted from January 1, 2000, through December 31, 2015, using Danish nationwide registries and the Eastern Danish Heart Surgery Database. Patients who developed POAF after isolated left-sided heart valve surgery (bioprosthetic aortic or mitral valve replacement and/or aortic or mitral valve repair) from 2000 through 2015 were included. These patients were matched with patients with nonsurgical NVAF in a 1:3 ratio by age, sex, heart failure, hypertension, diabetes, a history of thromboembolism, ischemic heart disease, and year of diagnosis. Data analyses took place from January to March 2019. Main Outcomes and Measures: Rates of thromboembolism. Results: Of the 1587 patients who underwent isolated left-sided heart valve surgery, 741 patients (46.7%) developed POAF during admission. Of the 712 patients with POAF who were eligible for matching, 675 patients were matched with 2025 patients with NVAF and made up the study population. In the matched study population, the median age was 71 (interquartile range, 65-77) years, and 1600 (59.3%) were men. Oral anticoagulation therapy was initiated within 30 days postdischarge in 420 patients with POAF (62.9%) and in 1030 patients with NVAF (51.4%). The crude incidence rates of thromboembolism were 21.9 (95% CI, 17.4-27.6) and 17.7 (95% CI, 15.2-20.6) events per 1000 person-years for patients with POAF and patients with NVAF, respectively. In the adjusted analysis, the long-term risk of thromboembolism was similar in patients with POAF and NVAF (hazard ratio, 1.22 [95% CI, 0.88-1.68]). Oral anticoagulation therapy during follow-up was associated with a lower risk of thromboembolic events in patients with POAF (hazard ratio, 0.45 [95% CI, 0.22-0.90]) as well as patients with NVAF (hazard ratio, 0.63 [95% CI, 0.45-0.87]) compared with no anticoagulation therapy. Conclusions and Relevance: New-onset POAF after isolated left-sided heart valve surgery was associated with a similar long-term risk of thromboembolism as NVAF. These data warrant studies addressing the role of anticoagulation therapy in POAF after left-sided heart valve surgery.