Stab injuries of the spinal cord are rare. We report a case of a 22-year-old male who was hospitalized because of a spinal cord injury resulting from a stab wound in the posterior thoracolumbar area. On admission, the patient had 2/5 muscle strength of the right leg (monoparesis) and hypoesthesia below the L1 level. Radiological investigation revealed the retained tip of a knife that penetrated the spinal canal at the T12 level. An urgent right T12 hemilaminotomy was performed and retained knife fragment was removed. Six months after operation, the motor deficit had completely improved although hypoesthesia was still present. Surgery should be considered as the first-line treatment in cases of incomplete injuries of the spinal cord with retained metallic object.