Intradiploic inclusion cysts are exceedingly rare in the pediatric population. The authors present a 16-year-old male patient who presented with a large growing calvarial mass with a preoperative diagnosis of fibrous dysplasia based on radiologic imaging. Craniectomy followed by autogenous reconstruction was performed. Histopathological examination revealed a relatively small inclusion cyst of the intradiploic space, surrounded by reactive bone. This patient demonstrates a highly unusual presentation of a rare entity, and the authors discuss the diagnosis and management of intradiploic inclusion cysts.