Four patients presented with rare spinal subdural hematoma (SDH) occurring after intracranial aneurysm surgery and manifesting as postoperative back pain. Magnetic resonance imaging performed from 4 to 11 days after the operation showed acute or subacute thoracolumbar SDH. No patient had risk factors for bleeding at this site (e.g., lumbar puncture, coagulation abnormality). Overdrainage of the cerebrospinal fluid (CSF) was performed for brain retraction during the operation in all four cases. Computed tomography performed during the postoperative period showed a suspicious tentorial subdural hemorrhage in Case 1 and an interhemispheric subdural hemorrhage in Case 3. All four patients received conservative management and their lumbago improved. We hypothesize that CSF hypotension due to overdrainage of CSF and downward migration of intracranial SDH under the influence of gravity were involved in the formation of spinal SDH.