Spinal subarachnoid hematoma is a very rare event, occurring exclusively in the thoracic or lumbar region. We report the first recorded case of spontaneous subarachnoid hematoma in the high cervical spine presenting as Brown-Séquard's syndrome. A 57-year-old woman with no prior problems suddenly presented with Brown-Séquard's syndrome at the C1-2 disc space level following occiput neck pain. Her consciousness was clear and urinary retention was not observed. Lumbar puncture revealed no evidence of hemorrhage. Neuroradiological evaluation, including myelography, CT myelography, and MRI, demonstrated a defined hematoma in the cervical subarachnoid space at the C1-2 level. Angiographical study yielded negative findings. The patient's neurological state remained unchanged for the following 6 days. On the 7th day from the onset, a C1 and C2 laminectomy was performed. A defined clot was found after incising the intact dura matter and arachnoid membrane. This clot was easily aspirated except for a small part which was found attached to a pial vessel on the dorsal surface of the spinal cord. No underlying pathology other than coagulated blood was confirmed. Three months postoperatively, she had no neurological deficits. The clinical course of spontaneous spinal subarachnoid hematoma varies according to the rapidity and severity of hematoma formation. An immediate and precise diagnosis using multimodal neuroimagings is vital because decompressive surgery can dramatically ameliorate the neurological sequelae.