Atrial fibrillation (AF) is the most common sustained dysrhythmia in the elderly population. It is estimated to affect more than 30 million people worldwide. AF occurs when abnormal electrical impulses start to activate in the atria and override the heart's natural pacemaker, which can no longer control the heart's rhythm. Since atrial contractility is impaired in AF, blood flow in the atria becomes stasis over time and causes thrombus formation. This thrombus causes the risk of embolism and causes complications such as stroke. Therefore, it is a fundamental cause of cardiovascular mortality and morbidity. The diagnosis of AF is usually made with the help of electrocardiography (ECG). The absence of P waves in ECG and irregular R-R interval is sufficient for diagnosis. AF is most commonly associated with advanced age, hypertension, diabetes mellitus, thyroid dysfunction, obesity, alcohol use, physical inactivity, and underlying ischemic heart diseases. As well as to all these usual risk factors, electrolyte disorders and mineral deficiencies also play an essential role in the etiology of AF. Previous studies have clearly demonstrated that serum electrolyte changes have a role in the etiology of AF. These include electrolytes such as serum magnesium, calcium, potassium, and chloride. However, there is not enough information in the literature about the effects of trace elements on AF. Selenium is a trace element that plays an important role in many systems in the human body. It has a vital role in inflammation, regulation of antioxidant reactions, and fibrosis of tissues in both animals and humans. It is known that selenium deficiency causes many cardiovascular diseases such as heart failure, coronary artery disease, and arrhythmia. Our study aimed to compare serum selenium levels in newly diagnosed AF patients with the healthy control group.