An 87-year-old woman presented with an atypical case of acute subdural hematoma (ASDH) manifesting as disturbance of consciousness and left hemiparesis. Computed tomography revealed a high density lentiform lesion in the right convexity, which was thought to be acute epidural hematoma preoperatively. Emergent decompressive craniotomy revealed an encapsulated solid fresh clot in the subdural space and a bleeding small cortical artery under the clot. The arachnoid membrane and the brain parenchyma were intact, and no other abnormal feature such as aneurysm or arteriovenous malformation was observed. The encapsulated ASDH was removed en bloc and the patient fully recovered. Histological examination confirmed that both the outer thicker and the inner membrane were fibrinous single structures without vasculature. The red blood cells constituting the clot in the capsule maintained their cell structure. The reported pathological mechanisms of lentiform ASDH are adhesion of the arachnoid membrane and the dura mater or intracapsular bleeding from sinusoidal vessels in the outer membrane of the chronic subdural hematoma. However, in our case, the arachnoid membrane had not adhered to the dura mater and the capsule was a fibrinous single structural membrane without vasculature, which probably resulted from a previous hematoma due to initial bleeding from the cortical artery. The possible mechanism in our case was that the re-bleeding dissected and flowed into the fibrinous single structural membrane, resulting in formation of the lentiform ASDH.