Primary testicular osteosarcoma is an extremely rare malignancy. To date, only two cases have been reported. Here, we report a third case of primary testicular osteosarcoma complicated with hydrocele. A 78-year-old man presented with right scrotal swelling. Ultrasonography revealed hydrocele and a testicular heterogeneous solid mass with focal calcification in the right testis. Right inguinal orchiectomy revealed a pure intratesticular osteosarcoma. Retroperitoneal lymph-node dissection revealed no metastasis. Thorough sampling of the tumor failed to show any additional histological components. It is unlikely that our case arose from teratoma or mixed sex-cord/stromal tumor, because no other neoplastic elements were identified in whole sampling of the tumor. He remained well without evidence of disease 44 months after operation. This case illustrates that primary pure testicular osteosarcoma may be associated with a favorable prognosis.