An asymptomatic 42-year-old man was diagnosed with a posterior mediastinal mass, most likely a nonfunctioning, benign, neurogenic tumor for which thoracoscopic surgery was initially indicated. However, the systemic blood pressure rapidly increased to a critical level immediately after starting the surgical manipulation of the tumor, which was suggestive of a hyperfunctioning pheochromocytoma. The tumor was removed after controlling the blood pressure and was histologically diagnosed as a pheochromocytoma. The patient had an uneventful course, and the tumor was proven to be sporadic through further postoperative investigations. The possibility of extraadrenal pheochromocytoma should be considered in the preoperative diagnosis of an intrathoracic paraaortic tumor, even in an asymptomatic patient.