A 13-year-old girl presented with dyspnea and chest pain. Chest radiography showed a massive left pleural effusion. Computed tomography revealed a tumor of the fourth rib. A large bloody effusion was drained. Her anemia worsened (hemoglobin: 4.8 g/dl), and hemorrhagic shock ensued. An emergency thoracotomy was performed. Bleeding from the ruptured tumor was identified. The fourth rib, the tumor, and the adjacent tissues were resected. Histopathologic examination revealed a ruptured primary osteosarcoma of the rib with pleural dissemination.