Plasmacytoma is a malignant tumor of the plasma cells. Extra-medullary plasmacytoma is rare and with an even lower incidence appears as a primary tumor of the stomach. Initial onset of the disease in the upper gastrointestinal tract is reported in the literature as just second to primary plasmacytomas of the head and neck system. The presenting symptoms are related to the organ involved and systemic symptoms can be weight loss, pain, bleeding and even fever. As this is a rare disease, there is no standard treatment and patients undergo endoscopic resection or chemotherapy with or without additional radiation. The prognosis of the disease depends on the possible future diagnosis of multiple myeloma which can be up to 50% within only a few years. We hereby report a case of a male patient with a past locally advanced breast cancer who was on prolonged adjuvant hormonal treatment. He developed a new symptom of melena and underwent a thorough evaluation including imaging and repeated biopsies from a large gastric lesion. The results were inconclusive mainly because of the differential diagnosis between breast cancer metastases and a new second primary malignancy. In view of a clinical deterioration and lack of diagnosis, an operation of radical gastrectomy was eventually performed only to surprisingly diagnose a rare hematologic disease of the stomach - gastric plasmacytoma. This diagnosis is rare in itself, especially having his previous male breast cancer and maternal multiple myeloma. The diagnostic procedure in this case had also provided the full treatment for his illness.