Abstract Objectives: To determine the incidence, risk factors, and outcomes of new-onset atrial fibrillation (NOAF) in a cohort of critically ill patients with coronavirus disease 2019 (COVID-19). Methods: We conducted a retrospective study on patients admitted to the intensive care unit (ICU) with a diagnosis of COVID-19. NOAF was defined as atrial fibrillation that was detected after diagnosis of COVID-19 without a prior history. The primary outcome of the study was the effect of NOAF on mortality in critically ill COVID-19 patients. Results: We enrolled 248 eligible patients. NOAF incidence was 14.9% (n=37), and 78% of patients (n=29) were men in NOAF positive group. Median age of the NOAF group was 79.0 (interquartile range, 71.5-84.0). Hospital mortality was higher in the NOAF group (87% vs 67%, respectively, p=0.019). However, in multivariate analysis, NOAF was not an independent risk factor for hospital mortality (OR 1.42, 95% CI 0.40–5.09, p=0.582) Conclusions: The incidence of NOAF was 14.9% in critically ill COVID-19 patients. Hospital mortality was higher in the NOAF group. However, NOAF was not an independent risk factor for hospital mortality in patients with COVID-19. Keywords: Atrial fibrillation, critical care, intensive care unit, COVID-19, mortality, hospital mortality