Gastric adenocarcinoma developing concomitantly with a gastric lymphoma of the mucosa-associated lymphoid tissue (MALT) type is rare. Herein, we report a case with a synchronous primary gastric MALT lymphoma and an early adenocarcinoma of the stomach. A 72-year-old patient with initial presentation of weight loss was found with endoscopy to have a large tumor mass in the gastric body. Pathologic examination of biopsies revealed a low-grade MALT lymphoma for which chemotherapy with cyclophosphamide, vincristine, and prednisolone was administered. A gastric adenocarcinoma was found at a different site in the of stomach 3 months after cessation of chemotherapy when there was still residual MALT lymphoma in the stomach. The presence of double neoplasms was established preoperatively. The patient underwent a proximal gastrectomy. Infection with Helicobacter pylori (H. pylori) was detected once in the repeated endoscopic gastric biopsies. The occurrence of both gastric MALT lymphoma and gastric adenocarcinoma was reviewed and the association of H. pylori infection with both malignancies is discussed.