The authors report the case of a 25-year-old man with a thoracolumbar extradural arachnoid cyst who underwent expansive duraplasty. Symptoms, preoperative magnetic resonance imaging features, and intraoperative findings suggested the involvement of entrapment neuropathy in the manifestation of symptoms. To the authors' knowledge, this case represents the first evidence that expansive duraplasty can achieve complete resolution of the symptoms in a patient with a spinal extradural arachnoid cyst. The results indicate that duraplasty may be an alternative option in cases in which complete resection of the lesion is difficult and widening of the dural sac is necessary at surgery.