A 59-year-old man with a history of melena and upper abdominal pain was referred to our hospital. An upper endoscopy was performed, and a gastric ulcer was found bordering the antrum and stomach body. Multiple biopsies from the lesion showed monoclonal plasmacytic infiltration of the mucosa, consistent with the diagnosis of plasmacytoma. Helicobacter pylori was also identified. Triple therapy failed and quadruple therapy eradicated the H. pylori, confirmed by repeated biopsies. Healing of the gastric lesion followed the treatment. Multiple biopsies from the scar and the entire stomach showed complete regression of the plasmacytoma. The association between gastric plasmacytoma and H. pylori is discussed.