Spinal subarachnoid hematomas are rare causes of spinal cord compression. They are usually associated with the use of anticoagulants, lumbar puncture, arteriovenous malformation (AVM), spinal artery aneurysm, etc. They may also occur spontaneously ; however, the incidence of this is rare. Herein, we report a rare case of spontaneous spinal subarachnoid hematoma in which emergency surgery was necessary. The patient was a 54-year-old woman who presented with sudden thoracodorsal pain and mild motor weakness of both lower limbs at a local hospital. Because the lower limb weakness progressed and bladder dysfunction was noticed, she was referred to our department on the third day after onset. CT and MR images revealed an intradural lesion compressing the spinal cord from the left ventral position at thoracic vertebrae (T) 2 through 9. Head CT revealed a small amount of SAH in the peripheral sulci. On spinal angiography, no clearly abnormal blood vessels were observed. Emergency surgery was performed on the day of admission. During surgery, we performed a left unilateral laminectomy from T4 to T8. No lesions were observed outside the dura mater or in the subdural cavity, and a hematoma was identified under the arachnoid membrane. Upon removing the hematoma, no abnormal blood vessels, tumors or other lesions were found. After surgery, the neurological symptoms improved. On postoperative days 4 and 5, mild disorientation was noted, but the patient recovered thereafter. CT and MRI performed 9 days after surgery revealed a hemorrhagic infarction in the right parietal lobe, which was presumably due to vasospasm. On day 10 after surgery, the patient was alert and able to walk with crutches, and was therefore transferred to a rehabilitation hospital. Spinal subarachnoid hematoma should be included in the diagnosis of thoracodorsal pain. If the patient's neurological status progressively deteriorates, early surgical treatment is recommended.