We present a unique case of a satellite pleural-based thymoma. The patient is a 66-year-old Caucasian female with a history of a left pericardial soft tissue mass. She had been asymptomatic. Chest radiograph incidentally revealed an acute increase in the size of the mass. CT scan identified a 5.6 X 5.2 X 4.2 cm mediastinal mass in the left infrahilar region along the left lateral pericardium. Positron emission tomography (PET) scan showed the mass had an increased F18 FDG uptake with standardized uptake value (SUV) of 7.2. Left thoracotomy resected a 81g, 6 X 5.5 X 5.0 cm tan-pink well-encapsulated pedunculated mass displacing the left phrenic nerve. The mass was under the parietal pleura and not attached to the pericardium. Immunohistochemical profile identified the tumor as a thymoma, B1 type. Thymomas are relatively rare in the United States, pleural-based thymomas even more so. Early detection of thymomas is critical to avoid late-stage growths. Pericardial involvement of thymomas increases risk of pericardial effusion, tamponade and a complicated thymectomy. Pleural-based thymomas can result in diaphragmatic paralysis secondary to phrenic nerve involvement.