The gastric plasmacytoma represents a rarely encountered, so that, the diagnosis criteria, the medical attitude and the evolution of this disease are less coded. It is known the fact that this tumours, extramedullary plasmacytomas with gastric localization, develop in soft submucous tissue, therefore it's very difficult or even impossible preoperative diagnosis through an upper endoscopy including biopsy. We present the case of a 54 years old patient, with no significant pathological personal record, that was admitted in our Clinic for having melenic dark stools which have repeated within a 6 months period prior to this hospitalisation. Repeated gastric mucosal samples taken in another medical unit shown normal aspects, contrary to all imaging testing (e.g. ultrasonography, upper endoscopy and barium swallow) which revealed a tumor mass of considerable extent at the level of the gastric corpus. First-line therapy was gastric resection followed by radiotherapy, because it is well known that the tumour is highly radiosensitive. The patient's postoperative course was satisfactory, showing no signs or recurrence 6 months after surgery at upper endoscopy and CT-scan.