Accessory liver of the thoracic cavity is usually asymptomatic, and its incidental detection is extremely rare. In this study, an unusual case of an accessory liver lobe of the thoracic cavity in a 26-year-old woman is described. A chest roentgenogram and thoracic computed tomographic scan revealed a mass in the left thoracic cavity. Left posterolateral thoracotomy was performed by removing a 10 × 8 × 5 cm3 mass separated from lung. The arterial and venous supply of the mass originated from the abdomen. The diaphragm was found to be intact. The pathologist reported a normal hepatic tissue. This report presented a very rare occurrence of accessory liver in the thoracic cavity.