Osteosarcoma is a very rare tumor of the urinary bladder, with only 28 well-documented cases reported in the literature to date. The tumors have characteristically been large and polypoid and most commonly have been located in the trigone. An abdominal mass has occasionally been palpated, and rectal examination has frequently revealed a mass in the region of the bladder. The prognosis for this tumor is very poor. In this report, a 75-year-old man with hematuria was found to have a primary osteosarcoma of the urinary bladder with superficial invasion. Radical cystoprostatectomy and ileal conduit urinary diversion were performed. During surgery, a synchronous mucinous adenocarcinoma of the ascending colon was found, and a right hemicolectomy was performed. Prostatic adenocarcinoma was discovered from the histology. The patient died 7 months after surgery with systemic spread of the osteosarcoma. Osteosarcoma of the bladder should be distinguished from other bladder tumors that may be associated with bone formation, such as carcinosarcomas and transitional cell carcinomas with osseous metaplasia of their stroma, both of which have a better prognosis than osteosarcoma.