Abstract Background The COMPASS trial enrolled patients with coronary artery or peripheral artery disease, but excluded patients requiring oral anticoagulation. Compared to aspirin alone, rivaroxaban in addition to aspirin reduced a composite of cardiovascular death, stroke or myocardial infarction. Purpose This study aimed to explore the clinical significance of a new diagnosis of atrial fibrillation (AF) during follow-up. Methods New AF was identified from unplanned hospitalization, study drug discontinuation and adverse event reports. The associations between new-onset AF, clinical characteristics and outcomes in COMPASS participants were studied using time-updated regression modeling. Event rates were reported as per 100 patient-years (following a new diagnosis of AF vs. from randomization until the end-follow-up or new AF, whichever came first). Results During a mean follow-up of 23 months, 655 of 27,395 participants (2.4%) were newly diagnosed with AF. Advanced age, male sex, white race, higher body mass index, higher systolic blood pressure, heart failure and prior myocardial infarction were independently associated with new-onset AF. The rates of permanent discontinuation of study drug were 83.8% following a new diagnosis of AF, compared to 17.5% in participants without/before new AF. Compared to participants without/before a new diagnosis of AF, participants with new-onset AF were at increased risk of a composite of cardiovascular death, stroke or myocardial infarction (8.9 vs. 2.5 per 100 patient-years, hazard ratio [HR] 4.49, 95% confidence interval [CI] 3.37-6.00) (Figure 1). Risk increases with new-onset AF were also observed for hospitalization for heart failure (9.1 vs. 1.1 per 100 patient-years, HR 6.35, 95% CI 4.59-8.80) (Figure 2) and major bleeding (4.1 vs. 1.3 per 100 patient-years, HR 2.93, 95% CI 1.99-4.33). Conclusion In COMPASS, a new diagnosis of AF during trial follow-up was associated with a marked increase in risk of adverse outcomes.Composite outcome after new-onset AFHeart failure after new-onset AF