Lipomatous hypertrophy of the interatrial septum is a rare, but increasingly recognized, anomalous developmental or neoplastic lesion of the heart. This entity was first described in 1964 at autopsy and is identified before death based on its distinctive characteristics on echocardiography, computed tomography, and magnetic resonance imaging. Although it is often asymptomatic, the mass has been associated with supraventricular arrhythmias and sudden death. In rare patients who experience intractable symptoms, surgical excision of the lesion may provide relief. Therefore, lipomatous hypertrophy of the interatrial septum is of interest to the pathologist when a cardiac mass is received for evaluation or at autopsy when a patient has experienced sudden death from an unknown cause.