Background: Atrial fibrillation (AF) and atrial flutter (AFL) are cardiac arrhythmias associated with cardiovascular morbidity. Physical activity (PA) can trigger AF and AFL recurrence, but can also improve physical functional capacity in this patient group. Guidelines do not include concrete recommendations regarding PA for this patient group. Objective: To assess the impact of the level of PA on risk of serious adverse events (SAEs) in patients with AF and/or AFL treated with catheter ablation. Methods: A prospective cohort study including 462 patients with AF and/or AFL treated with catheter ablation from the CopenHeart Survey. The International Physical Activity Questionnaire (IPAQ) was used to explore patients’ self-reported level of PA. SAEs were identified in the Danish National Patient Register and the Danish Civil Registration System one year after study onset. Cox regression analysis was carried out to assess the risks of SAE. Results: During the one-year follow-up period, 98 patients (21.8%) experienced at least one SAE. Patients with a moderate-high PA level had a 36% lower risk of experiencing SAEs during the follow-up period, compared to patients in the low PA group, after adjusting for confounders. Conclusion: A moderate-high vs. low level of PA was found to be associated with a lower incidence of SAEs in patients undergoing AF and/or AFL ablation.