The objective of this retrospective cohort study was to describe the incidence of paroxysmal atrial fibrillation and to determine its risk factors and effect on outcome in critically ill patients with sepsis. The study included 81 patients with sepsis admitted to an intensive care unit. In all, 25 patients (31%) developed paroxysmal atrial fibrillation. Advanced age, history of paroxysmal atrial fibrillation, higher severity of illness at intensive care unit admission, and lower left ventricular ejection fraction were risk factors for paroxysmal atrial fibrillation. Multiple logistic regression analysis showed that paroxysmal atrial fibrillation was independently associated with 28-day mortality (odds ratio = 3.284; 95% confidence interval, 1.126-9.574). The incidence of paroxysmal atrial fibrillation is high in critically ill patients with sepsis. It occurs more frequently in patients with advanced age, history of paroxysmal atrial fibrillation, high severity of illness, and lower left ventricular ejection fraction and is associated with increased mortality.