Congenital anatomic anomalies and variations are frequent in the thoracic outlet and may be associated with clinical symptoms. Arterial thoracic outlet syndrome (TOS) is characterized by subclavian artery compression and vascular pathology, almost always in the presence of a bony abnormality. We describe here a patient with arterial thromboembolism following a fall on the outstretched arm, who was found to have subclavian artery stenosis and post-stenotic dilatation in the absence of a bony abnormality. Surgical exploration revealed a previously undescribed anomaly in which the subclavian artery passed through the costoclavicular space in front of the anterior scalene muscle, where it was subject to bony compression between the first rib and clavicle. Successful treatment was achieved by scalenectomy, first rib resection and interposition bypass graft reconstruction of the affected subclavian artery. This newly acknowledged anatomical variant adds to our understanding of the diverse factors that may contribute to development of TOS.