Intraosseous or intradiploic arachnoid cysts of the cranial vault are extremely rare lesions, representing herniations of the arachnoid membrane through a dural and inner table defect. A 35-year-old woman presented with progressive headaches and a slowly enlarging, non-tender left parietal swelling without any history of trauma. Imaging revealed a left parietal intraosseous cystic lesion with CSF-like signal intensity, thinning of the outer table, and communication with the subarachnoid space. Surgical exploration confirmed multiple small openings in the outer table with CSF outflow and an underlying dural defect. The cyst was excised, the dura repaired watertight, and the cranial vault reconstructed. The postoperative course was uneventful, and follow-up imaging at three months showed no recurrence. This case highlights an exceptional presentation of an intraosseous arachnoid cyst with dural communication. Recognition of this rare entity is essential for accurate diagnosis and proper surgical management.