Lipomatous hypertrophy of the interatrial septum is a relatively common disorder that occasionally presents as cardiac mass. Here, we present a case of extreme lipomatous hypertrophy up to 4 cm in diameter and extending into the epicardium, posterior wall of the right atrium and the atrioventricular sulcus. Diagnosis was confirmed by transesophageal echocardiography and cardiac magnetic resonance imaging.