Subdural hematoma is a serious but rare complication of spinal anesthesia. A 70-year-old woman patient underwent elective total knee replacement under spinal anesthesia. At 4 days postoperatively, the patient complained of headache and vomiting. Brain computed tomography revealed an acute-on-chronic subdural hematoma with midline shift. The patient recovered completely after surgical decompression. We report a patient with an undiagnosed chronic subdural hematoma, who developed acute-on-chronic subdural hematoma after spinal anesthesia.