Abstract Background There is paucity of information about major adverse cardiovascular event (MACE) related to physical activity change after early rhythm control (ERC) after new-onset atrial fibrillation (AF). We investigated the association between physical activity after ERC and clinical outcomes in new-onset AF patients. Methods This study was performed based on the Korean National Health Insurance Service database. We included adults with new-onset AF between 2009 and 2018. Patients who received national health screening examination before and after AF diagnosis within 2-year were finally included (n=161,379). Among these, 48,243 patients were received rhythm control within 2-year after AF diagnosis. Patients were categorized into 4 groups as follows: persistent non-exerciser, new exerciser (after AF diagnosis and ERC), exercise maintainer, and exercise quitter (after AF diagnosis and ERC). The primary outcome was MACE defined as the composite of ischemic stroke, myocardial infarction, and hospitalization for heart failure. Multivariable Cox analysis was performed. Results Among 48,243 patients receiving ERC, 63% (n=30,540) was persistent non-exerciser, 13% (n=6260) was new exerciser, 11% (n=5102) was exercise maintainer, and 13% (n=6341) was exercise quitter. After multivariable adjustment, compared to persistent non-exerciser, new exerciser and exercise maintainer with ERC showed lower risk of MACE by 10% and 19%, respectively (adjusted hazard ratio [aHR], 95% confidence interval [95% CI], 0.903 [0.802-0.994] for new exerciser, and 0.810 [0.725-0.906] for exercise maintainer) (Table 1). However, exercise quitter did not show significant benefit compared to persistent non-exerciser (Table 1). To assess the synergistic effect of ERC and starting or maintaining physical activity, we performed exploratory analysis including patients who did not receive ERC. Compared to persistent non-exerciser without ERC, persistent non-exerciser with ERC was associated with a significantly lower risk of MACE (0.832 [0.797-0.868], Table 2). Patients who received ERC and started or maintained physical activity with ERC showed greatest risk reduction for MACE by 27% and 36%, respectively (0.732 [0.668-0.802] for new exerciser and 0.641 [0.576-0.713] for exercise maintainer) (Table 2). Conclusion Even in context with beneficial effect of ERC in patients with new-onset AF, maintaining or starting regular physical activity after ERC might bring greater reduction on the risk of MACE. Physician should emphasize healthy lifestyle behavior with rhythm control in patients with new-onset AF.