We report an unusual case of left atrial auricular abscess which was successfully treated by surgical resection, treatment with antibiotics and mediastinal irrigation. A 9-year-old female patient with previous history of urinary tract infection was admitted because of persistent fever. Echocardiography and magnetic resonance imaging revealed massive pericardial effusion and a mass lesion at the left upper cardiac border. Pericardiocentesis isolated Staphylococcus aureus on culture. The patient underwent mass removal under cardiopulmonary bypass. The mass was located within the left atrial auricular wall with fibropurulent abscess formation inside. Postoperative mediastinal irrigation was performed. Pathologic examination of the mass showed organized thrombi with chronic fibrosing mural endocarditis. She discharged on the 21st postoperative day without complication.