Aim: The prevalence of atrial fibrillation (AF) in patients with myocardial infarction (MI) ranges widely and has been reported to be as high as 21%. However, the demographic, clinical, and angiographic characteristics of AF patients with de novo MI is unclear. The aim of this study was to investigate the prevalence of patients presenting with de novo MI with AF. Methods: The study was performed as a sub-study of the MINOCA-TR (Myocardial Infarction with Non-obstructive Coronary Arteries in Turkish Population) Registry, a multicenter, cross-sectional, observational, all-comer registry. MI patients without a known history of stable coronary artery disease and/or prior coronary revascularization were enrolled in the study. Patients were divided into AF and Non-AF groups according to their presenting cardiac rhythm. Results: A total of 1793 patients were screened and 1626 were included in the study. Mean age was 61.5 (12.5) years. 70.7% of patients were men. Prevalence of AF was 3.1% (51 patients). AF patients were older [73.4 (9.4) vs. 61.0 (12.4) years, p<0.001] than non-AF patients. The proportion of women to men in the AF group was also higher than in the non-AF group (43.1% vs. 28.7%, p=0.027). Dramatically, the minority of patients were previously diagnosed with AF (14 patients, 27.4%) and only 1 out of every 5 AF patients (10 patients, 19.6%) was using oral anticoagulants (OAC). Conclusions: AF prevalence in patients presenting with de novo MI was lower than reported in previous studies. The majority of AF patients did not have any knowledge of their arrhythmia and were not undergoing OAC therapy at admission, emphasizing the vital role of successful diagnostic strategies, patient education, and implementations for guideline adaptation.