Subdural hematoma (SDH) is bleeding between the dura mater and the brain surface, often visualized as a crescent-shaped lesion on computed tomography (CT). However, some SDHs can appear as a biconvex hyperdense lesion mimicking an acute epidural hematoma. Encapsulated acute SDH (EASDH), a rare subtype characterized by a fresh hematoma within the subdural fibrous capsule, presents with similar CT findings. Herein, we describe an autopsy case of EASDH and discuss its etiology based on the case findings and available literature. The autopsy revealed a rupture of the bridging arteries as the SDH bleeding source, confirming the development of an acute-on-chronic spontaneous SDH. To our knowledge, this is the first autopsy report of EASDH and the third case of spontaneous SDH due to bridging-artery rupture.