Lipomatous Hypertrophy of the Interatrial Septum (LHIS) is an unusual condition that can be presented as a mass-like bulge that typical spares of the fossa ovalis. We describe a 73-year-old overweight woman with dyspnea on exertion and two episodes of paroxysmal atrial fibrillation for the last eight months. A big sessile mass was detected in the interatrial septum measuring 3.7 x 4.7cm during transesophageal echocardiography. The patient underwent resection of the mass and pericardial patch reconstruction of the defect with significant improvement of her clinical status during the follow-up period. The histopathological analysis suggested LHIS.