There are some intraspinal cystic lesions presenting with myelopathy. We report a case of myelopathy caused by a possible thoracic interdural ganglion cyst. A 70-year-old man with subacute bilateral lower extremity numbness, muscle weakness, and gait disturbance presented to our hospital. Magnetic resonance (MR) images showed a cystic lesion which compresses the left dorsolateral intraspinal space of T2-3. During surgery, a ganglion cyst was found without adhering to the periphery of the epidural space. The capsule and contents were removed. He showed postoperative improvement in activities of daily living. A postoperative pathological diagnosis of ganglion cyst was made. The development mechanism of thoracic interdural ganglion cysts is unknown. To our knowledge, this is the first report of this disease. Surgery improved symptoms of a patient with myelopathy caused by thoracic interdural ganglion cysts. This must be considered as one of the cystic lesions presenting with myelopathy.