A 51-year-old woman presented with lumbar backache leading to the preoperative diagnosis of a right solid adnexal mass with calcification on computed tomographic scan. Histological examination revealed a right ovarian luteinized thecoma characterized by extensive calcification and metaplastic ossification. Osteoblasts and osteoclasts surrounded the surface of the heterotopic bone. Haversian canals were occasionally identified in the bony trabeculae. Ossifying ovarian neoplasms are extremely rare and this case is the first to demonstrate the osseous metaplasia in ovarian luteinized thecoma.