A 52-year-old man with long-standing craniofacial polyostotic fibrous dysplasia (FD) and no history of prior radiation therapy developed a spontaneous right temporal bone osteosarcoma. Such spontaneous sarcomatous degeneration of FD is rare, particularly in the calvarium/skull, where, to our knowledge, only six prior cases have been reported in the literature. We report this case because it is a rare entity with well-documented CT and MR images, and to emphasize the importance of depicting imaging features of sarcomatous degeneration among the complex imaging findings of FD.