Atrial fibrillation (AF) is the most common cardiac dysrhythmia. Its prevalence, risk factors, course, and complications are not well described in critically ill trauma patients. This was a retrospective, single-center, cohort study at an academic, level 1 trauma center. Trauma patients >18 years, identified from the trauma registry and admitted to the intensive care unit (ICU), were sequentially screened for AF. A matched cohort was created by selecting patients consecutively admitted before and after the patients who experienced AF. Of 2591 patients screened, 191 experienced AF, resulting in a prevalence of 7.4%. There was no difference in injury severity score (ISS) between those with and without AF, but patients with AF had higher observed mortality (15.5% vs 6.7%, P < .001). Patients with a history of AF (n = 75) differed from new-onset AF (n = 106) in their mean age, 78.9 ± 8.4 versus 69.2 ± 17.9 years; mean time to AF onset, 1.1 ± 2.3 versus 5.2 ± 10.2 days; median duration of AF, 29.8 (1-745.2) versus 5.9 (0-757) hours; and rate of AF resolution, 28% versus 82.1%, respectively. Despite a higher ISS, Sequential Organ Failure Assessment and length of stay, the new-onset AF group experienced a similar rate of mortality compared to the history of AF group (14.7% vs 16.0%). Patients with AF had a higher mortality when compared to those in sinus rhythm. The course of AF in the new-onset AF group occurred later was shorter and was more likely to convert; however, these patients had a longer ICU stay when compared to those who had a history of AF.