Primary osteosarcoma originating from the ovary is an exceedingly rare, highly malignant tumor. Only a few cases have been reported in the past few decades. We describe a 50-year-old postmenopausal woman who presented with a large abdominal mass. The clinical diagnosis was malignant ovarian cancer. Her disease was aggressive; she had no response to systemic chemotherapy and died within 1 month of presentation. A definitive diagnosis of primary ovarian osteosarcoma was made by histopathological examination of autopsy specimens. Although rare, primary ovarian osteosarcoma should be considered in the differential diagnosis of a large, rapidly progressing pelvic mass in a postmenopausal woman. Early diagnosis provides hope of a complete surgical resection, which is currently the only promising treatment.