BACKGROUND Lymphoplasmacytic lymphoma (LPL) is a mature B cell lymphoma that mostly involves the bone marrow, spleen, and lymph nodes. Involvement of extramedullary sites is very rare and has not been reported as the primary site before. CASE REPORT A 47-year-old man presented with reflux symptoms. Gastroscopy revealed a 1.5-cm gastroesophageal junction (GEJ) polyp and oesophageal ulcer. A biopsy was performed and histopathology showed active chronic inflammation with focal intestinal metaplasia and reactive epithelial changes. A CT abdomen showed eccentric thickening of the lower oesophagus and GEJ, with periesophageal, gastro-hepatic ligament, and coeliac lymph node (LN) enlargement. A laparoscopic biopsy showed no peritoneal disease. EUS showed a large ulcerated lesion in the GEJ and proximal stomach. Both were biopsied, showing squamous-columnar mucosa with edema and a population of plasma cells, small lymphocytes, and histiocytes. These expressed CD20, PAX5, CD79a, IgM, and were lambda light chain-restricted. Lymphocytes were negative for CD3, IgG, IgA, and IgD. The MIB-1 index was low. LPL was diagnosed. PET showed an increased uptake of the gastric cardia and GEJ. LNs were not metabolically active. Bone marrow was negative. Evaluation of MYD 88 mutational status failed. Serum immunofixation showed no paraprotein. These results led to a diagnosis of primary isolated LPL of the stomach. CONCLUSIONS Primary lymphoplasmacytic lymphoma may present as an isolated gastric tumor. This can be unassociated with a paraprotein in serum and increased lymphocyte/plasma cell populations within the bone marrow. Gastric LPL is rare. Physicians and pathologists need to be aware of this rare presentation.