Background: Atrial fibrillation (AF) is the most common arrhythmia seen in critically ill patients, which leads to mortality and morbidity. Purpose of this study was to assess the impact of new onset AF in critically ill patients.Methods: We have designed an observational study, with participants aged more than or equal to 18 years, having new onset AF in medical ICU from 1st January, 2024 to 30th September, 2024 with previous documentation of normal sinus rhythm.Results: 28 patients developed new onset AF during the study period in medical ICU. 42.8% was of cardiac origin, whereas 57.2 % was of non-cardiac origin. Mortality was higher among AF of non-cardiac origin. Mortality was higher among hemodynamically unstable patients. New onset AF and mortality in this group was directly proportional to SOFA (Sequential Organ Failure Assessment) score.