A primary metastatic malignant plasma cell tumor was presented in a 7-year-old, female, mixed breed dog. The dog was admitted to Veterinary Teaching Hospital at College of Veterinary Medicine, Seoul National University after suffering from depression, anorexia, and dyspnea. The dog was euthanized due to poor prognosis. At necropsy, numerous, firm and tan round nodules, 1 to 2 cm in diameter were noted in the ribs. The nodules formed chain or were occasionally coalescing, causing osteolysis of the ribs. A large amount of clear fluid was present in the thoracic cavity. The spleen and liver were markedly enlarged and congested. Histologically, the nodular masses were diagnosed as plasma cell tumor which was believed to be originated from the rib. Evidence of metastasis was observed in the spleen, liver, and kidney.