A 60-year-old man presented with a traumatic leptomeningeal cyst manifesting as local tenderness in the right parietal region and local headache 2 years after head injury. Magnetic resonance imaging showed a small arachnoid cyst under bone defect. Dural and bone plasty were performed. Intraoperative examination found small and round defects of the dura and bone. Progressive headache was relieved after the surgery. This rare case of adult posttraumatic leptomeningeal cyst occurred within an unusually short period after trauma, and was associated with a small and round bone defect and small dural defect usually characteristic of congenital arachnoid cyst.