Brachial plexus magnetic resonance (MR) examination of an adult female synchronized swimmer, who was suffering from right-sided arm pain and weakness while making a stroke, showed an aberrant "subclavius posticus muscle" in the parascapular region on the right side. The muscle had an attachment anteriorly on the first rib and posteriorly on to the scapula. The belly of this aberrant muscle showed proximity to the brachial plexus. Presence of such an anomalous muscle has been recognized as a possible cause of neurovascular compression or thoracic outlet syndrome (TOS). Radiologists and surgeons should be familiar with this rare entity and consider it in the differential diagnosis of TOS.