There is a paucity of data on atrial fibrillation (AF) complicating acute coronary syndrome (ACS) in Arabian Gulf countries. Thus, we assessed the incidence of AF in patients with ACS in these countries and examined the associated in-hospital, 30-day, and 1-year adverse outcomes. The population comprised 7930 patients enrolled in the second Gulf Registry of Acute Coronary Events (Gulf RACE-2). Of 7930 patients with ACS, 217 (2.7%) had AF. Compared with patients without AF, patients with AF were less likely to be male (65.9 vs 79.1%) and were older (mean age 64.6 vs 56.6 years). Compared with patients without AF, in-hospital, 30-day, and 1-year mortality were significantly higher in patients with any AF (odds ratio [OR]: 2.7, 2.2, 1.9, respectively; P < .001) and in patients with new-onset AF (OR: 5.2, 3.9, 3.1, respectively; P < .001. In conclusion, AF in patients with ACS was associated with significantly higher short- and long-term mortality.