Vascular involvement in Behçet's disease is divided into venous and arterial thrombosis and arterial aneurysmal formation. Subclavian arterial aneurysm rarely occurs in Behçet's disease; however, when it does occur, it causes serious aneurysmal rupture and local complications such as nerve compression and arterial ischemia. We describe the case of a 39-year-old male who presented with neurologic symptoms and signs of brachial plexopathy and mediastinal mass caused by Behçet's subclavian arterial aneurysm. This case shows that the occurrence of brachial plexopathy should be considered a manifestation of Behçet's disease, and that Behçet's aneurysm should be considered in the differential diagnosis of upper mediastinal mass.