Anomalous muscle causes thoracic outlet syndrome (TOS). A 40-year-old man presented with numbness of the left upper extremity, similar to cervical spondylotic radiculopathy. He presented with a positive Bakody's test. However, magnetic resonance imaging showed no significant changes in the cervical spine and revealed an anomalous muscle adjacent to the left brachial plexus. We diagnosed the muscle as the cause of TOS and performed a resection, which resulted in symptomatic improvement.