Intrathoracic ribs are very rare congenital anomaly. We report a case of a third accessory intrathoracic rib located anterior to the normally developed right third rib in a 37-year-old man. Chest X-ray was not diagnostic. Multidetector computed tomography with 3D reconstructions was required for definitive diagnosis. These anomalies are usually clinically silent and detected incidentally by imaging studies. They should be kept in mind in the differential diagnosis of thoracic pathologies.