Osteosarcoma is a highly malignant soft tissue tumor. Bladder extra-skeletal osteosarcomas are very rare, and only few cases are reported in the English medical literature. This case is the third one of its kind to be reported consisting of a bladder diverticulum osteosarcoma. Compared to urothelial bladder carcinoma, urinary bladder osteosarcomas have a poorer prognosis. Biopsy and immunochemical staining are critical for the diagnosis. Treatment regimens vary based on the stage of the disease. However, choosing the best option for treatment is still unclear, due to the scarcity of cases available.