Background: Obesity is a risk factor for cardiovascular diseases, however evidence is mainly based on body mass index (BMI) analysis. The aim of this study was to estimate body composition using bioelectrical impedance analysis (BIA) and its prognostic value in patients with atrial fibrillation (AF) during one year of follow-up. Methods: Medical history, anthropometric and biochemical examinations, and BIA were performed in 120 inpatients with AF and 240 patients hospitalized due to other cardiovascular disorders. Results: Compared to the control group, the patients with AF had a significantly greater body mass, BMI, excess body mass in relation to ideal body mass, larger waist circumference, higher values of BIA parameters such as fat and fat-free mass, visceral adiposity, total body water (total water content in the body), metabolic rate and age, and lower percentage of skeletal muscle mass. Higher prevalence rates of obesity and sarcopenia were observed in the patients with AF compared to the control group, and in the patients with the paroxysmal form compared to those with the permanent form of AF. In logistic regression analysis, the percentage of fatmass was associated with a higher risk of AF (odds ratio, 1.10; 95% confidence interval, 1.05-1.15; p < 0.001). Conclusions: Body composition parameters were associated with the occurrence and form of AF in our study cohort. However, further studies are needed to clarify the relationships due to an imbalance in comorbidities.