A nine-year-old male intact Golden Retriever was presented to the Cornell University Hospital for Animals for suspected osteosarcoma of the left hind leg. The patient had a one-year history of hyperglobulinemia and a mass in the prescapular region that was removed two years prior without histopathology. The patient was evaluated by a referring veterinarian for being lame in the left hind leg for at least one month. Radiographs revealed extremely aggressive osteolytic lesions of the distal femur. Due to the high suspicion of osteosarcoma the patient was referred to the Cornell University Oncology service. Physical examination on presentation revealed a large, subcutaneous, freely moveable, non-painful mass in his left prescapular region and significant muscle atrophy of his left hind leg with pain on palpation. Additional diagnostics revealed a mild normocytic, normochromic non-regenerative anemia, hypoalbuminemia and hyperglobulinemia. Urinalysis revealed 2+ proteinuria, a urine specific gravity of 1.030, and no other abnormalities. Left hind limb radiographs confirmed the aggressive osteolytic lesions of the distal femur with a pathological fracture. Thoracic radiographs revealed a periosteal reaction of the first sternebra, an expansile lesion within the eighth rib, and a well-defined, round, soft tissue nodule in the left lung. Abdominal ultrasound showed multiple nodules within both kidneys, both testes, spleen, and liver. Amputation and histopathology of the left hind limb revealed a plasma cell infiltrate as well as a fibroblastic osteosarcoma. Serum electrophoresis revealed a monoclonal gammopathy. These tests confirmed a diagnosis of both multiple myeloma and osteosarcoma. Treatment with doxorubicin and carboplatin was initiated. Initial response to chemotherapy was encouraging, however after 4 months of treatment, progression of disease was noted with a systemic decline. The patient was lost to follow-up after recommendations were made to begin melphalan or palliative radiation treatment. This is the first described case of concurrent multiple myeloma and osteosarcoma in a dog.