An 8-year-old Shih Tzu developed abdominal pain and hyperglobulinemia. A round splenic mass was noted radiographically and sonographically. The patient was evaluated by fluorodeoxyglucose positron emission tomography coupled with computed tomography (FDG-PET/CT). There was no evidence of metastasis or bone marrow involvement on PET/CT images. The standardized uptake value (SUV) of the splenic mass was increased over the reference range (SUV = 4.83). The patient was diagnosed as splenic extramedullary plasmacytoma through immunohistopathologic study. After the splenectomy, the globulin level normalized and the patient is alive without complications.