The most common site for the metastasis of osteosarcoma is the lung, and other sites of metastases include the bone, lymph node, pleura and liver. Although unusual extrapulmonary metastases have been reported with the improvement of the therapeutic results for the primary lesions, they are exceptionally rare. We report here on a case of prostatic metastasis of an osteosarcoma of the proximal tibia, and this developed seven years after successful resection, and four years after resection of a pulmonary metastasis. Radical prostatectomy was performed, and histological examination demonstrated metastatic osteosarcoma. To the best of our knowledge, this is the first case of prostatic metastasis of osteosarcoma in the medical literature.