Spinal extradural arachnoid cyst is uncommon and rarely cause neural compression. We report a rare case of severe cord compression due to septated spinal extradural arachnoid cyst. A 35-year-old woman has developed back pain 3 months prior to her visit, but recently motor weakness and urinary incontinence occurred. Magnetic resonance images showed an extradural cyst posterior to the cord, which was flattened and displaced from T12 to L2. Urgent decompressive laminectomy and cyst removal was performed. Histopathological examination confirmed that cyst wall was formed by nonspecific fibrous connective tissue without a single-cell layer of inner arachnoid lining. Motor weakness and voiding difficulty were recovered completely after operation.