Spinal intradural arachnoid cysts (ACs) are found frequently in the thoracic region, and often extend over four or five vertebral levels. We present a 28-year-old patient who had a giant thoracic congenital intradural extramedullary AC (T1-T12) with a 10-month history of pain, paresthesia, paraparesis and gait ataxia. A T3 to T6 laminectomy was performed. After durotomy, the posterior wall of the AC was visualized compressing the spinal cord. We resected the cyst wall as widely as possible and connected the cyst to the subarachnoid space using a catheter. There were no postoperative complications. At 1-year follow-up, the patient presented with no motor deficits or pain, and had experienced progressive resolution of the gait ataxia. The treatment of giant intradural extramedullary ACs, especially for those that cannot be totally excised, should include generous fenestration and the insertion of a cyst-subarachnoid shunt.