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Background: Electrical cardioversion (CV) is essential in rhythm management of atrial fibrillation (AF). However, optimal timing of CV remains unknown. Hypothesis: Timing of CV in AF is associated with risk of adverse events. Methods: We analyzed the effect of AF episode duration on safety and efficacy of electrical CV in a multicenter, multicohort study exploring 4356 CVs in 2530 patients on oral anticoagulation. The composite adverse outcome included unsuccessful CV, acute arrhythmic complications, thromboembolic events, mortality, and AF recurrence within 30‐day follow‐up. Results: Study groups were stratified according to duration of index AF episode ( 30d), consisting of 1767, 516, 632, and 1441 CVs, respectively. CVs were unsuccessful in 8.5% (30d), respectively (P 5 seconds (0.7%) within 30‐day follow‐up was infrequent and comparable in the study groups. AF recurrence within 30 days after initially successful CVs was 29.8% (30d), respectively (P 48 hours was an independent predictor for the composite endpoint (OR: 1.49, 95% CI: 1.28–1.74, P < 0.01) in multivariate analysis. Conclusions: Optimal timing of CV for AF showed a J‐shaped curve, with fewest adverse outcomes in patients with CV performed 24 to 48 hours after onset of AF. In patients with rhythm‐control strategy, delaying CV >48 hours is associated with increased risk for adverse outcomes.