Background Mortality in critically ill patients with coronavirus disease 2019 (COVID‐19) is high, therefore, it is essential to evaluate the independent effect of new‐onset atrial fibrillation (NOAF) on mortality in patients with COVID‐19. We aimed to determine the incidence, risk factors, and outcomes of NOAF in a cohort of critically ill patients with 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 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 = .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 = .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. The incidence of NOAF is high in critically ill COVID‐19 patients. Hospital mortality is high in severe COVID‐19 patients with NOAF. Patients with NOAF are older and have co‐existing comorbidities.