A 66-year-old woman experienced a left femoral artery occlusion as a result of an adenocarcinoma embolization. A progressive tumor in the thoracic aorta was observed on a contrast-enhanced chest CT. The tumor was diagnosed as a metastatic lesion of unknown primary site and was thought to be the origin of the embolization. Metastasis to the wall of the thoracic aorta has not been previously reported, although arterial metastasis is common. We report a very rare case of a metastatic carcinoma to the thoracic aorta.