Aneurysm of the azygos vein is rare. We describe the case of a 51-year-old nonsmoking woman with a posterior mediastinal mass caused by a giant azygos vein aneurysm with subtotal thrombosis. Surgical resection of the azygos vein was offered to our patient as a treatment option owing to theoretical risks of rupture and pulmonary embolism. After taping the azygos vein proximally and distally, the aneurysm was resected with video-assisted thoracoscopy. Approximately 30 cases have been reported in the literature to date. Dynamic computed tomography and a videoassisted approach were useful for the diagnosis and treatment for this abnormality.