A 12-year-old English Cocker spaniel was referred because of front leg lameness due to a mass onthe right cranioventral thoracic wall. Radiographic examination showed that the limited external masshad a large intrathoracic component. The patient was staged, and based on the results of computedtomography, surgery was planned and the tumor was removed en bloc. An osteosarcoma with metastasisin the regional lymph nodes was diagnosed on histopathology. Consequently, chemotherapy wasinstituted. The disease-free period lasted four months, after which respiratory discomfort developeddue to pulmonary metastases. The dog was euthanized five months after surgery.This case illustrates that a sufficiently aggressive, multimodal treatment may prolong life expectancyin case of an advanced, malignant tumor.