ObjectiveTo summarize data on the prevalence/incidence, risk factors and prognosis of atrial fibrillation (AF) in patients with acute coronary syndromes (ACS).MethodsMEDLINE, Embase, and Web of Science were searched to identify all published studies providing relevant data through August 23, 2020. Random-effects meta-analysis method was used to pool estimates.ResultsWe included 109 studies reporting data from a pooled population of 8 239 364 patients. The prevalence rates were 5.8% for pre-existing AF, 7.3% for newly diagnosed AF, and 11.3% for prevalent (total) AF, in patients with ACS. Predictors of newly diagnosed AF included age (per year increase) (adjusted odds ratio [aOR]: 1.05), C-reactive protein (aOR: 1.49), left atrial (LA) diameter (aOR: 1.08), LA dilatation (aOR: 2.32), left ventricular ejection fraction <40% (aOR: 1.82), hypertension (aOR: 1.87), and Killip ˃ 1 (aOR: 1.85), p ConclusionOne in nine patients with ACS has AF, with a high proportion of newly diagnosed AF. AF, in particular newly diagnosed AF, is associated with poor short-term and long-term outcomes in patients with ACS.