A 62-year-old female was admitted with complaints of sudden proptosis of the left eye and severe left orbital pain without history of trauma or previous illness. Computed tomography and magnetic resonance imaging demonstrated a biconvex extraconal hematoma in the upper part of the left orbit. Surgical exploration of the orbit revealed the presence of a subperiosteal hematoma without a causative lesion. Such spontaneous subperiosteal hematoma of the orbit is extremely rare, but may result from an occult vascular disorder.