A 37-year-old female presented with a rare spinal subependymoma manifesting as progressive weakness of her right lower extremity over an 8-month period. She had a 10-year history of back pain and urinary disturbance. Magnetic resonance imaging showed diffuse enlargement of the spinal cord from T-2 to T-7 on the T₁-weighted images. The enlarged spinal cord was divided into two compartments by a vertical septum-like structure on the T₂-weighted images. The tumor occupied the right half of the thoracic spinal cord, and was totally removed through a laminectomy from T-2 to T-7. The histological diagnosis of the resected specimen was subependymoma. Subependymomas are slow-growing tumors usually found in the ventricular system. Spinal subependymomas are difficult to distinguish from other intramedullary spinal tumors based on neuroradiological findings. Subependymomas are surgically curable tumors, so if the tumor is well demarcated and a subependymoma is indicated, an attempt should be made to totally remove the tumor.