We describe a rare case of myocardial plasmacytoma staged and followed up with FDG PET/CT. A 72-year-old man was incidentally identified with a right ventricular apical mass, which was pathologically confirmed to be a plasmacytoma. A pre-treatment FDG PET/CT scan subsequently showed lesions not only in the right ventricle but also in the bones and mediastinal lymph nodes, which led to the change in treatment plan. Post-therapy PET scan revealed good response. This case demonstrates the value of FDG PET/CT in accurately staging unusually presented plasmacytoma and in monitoring response to treatment.