A 53-year-old male with a remote history of colon adenocarcinoma presented with weakness, severe anaemia and an actively bleeding ulcerated lesion in the stomach. An 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT showed FDG-avid masses in the stomach and mesentery, which were biopsied to reveal an unsuspected diagnosis of plasmacytoma. The original colon tumour pathology was identical and this prompted its re-evaluation to a primary colon plasmacytoma. The patient was treated with chemotherapy and a follow-up PET/CT scan showed complete resolution of the gastric and mesenteric masses. 18F-FDG PET/CT is useful in the restaging and follow-up of this very rare extramedullary plasmacytoma.