Although primary gastric malignant lymphoma accounts for slightly more than 10% of all lymphomas at extranodular sites, it is relatively rare clinically, representing only 1% of all malignant diseases of the stomach. In addition, most such diseases tend to be B-cell lymphoma, while T-cell lymphoma is extremely rare. We encountered a patient with primary gastric T-cell malignant lymphoma who, although demonstrating a very rare phenomenon, was negative for antihuman T-lymphotropic virus type 1 antibody. A 73-year-old man was admitted to the hospital with the chief complaint of upper abdominal pain. The primary lesion was a type 3 tumor located at the cardia to the posterior wall of the upper body of the stomach, which had invaded the tail of the pancreas and a part of the transverse colon. A total gastrectomy, pancreatosplenectomy, and partial resection of the transverse colon were performed. The surgical section contained a giant ulcerative lesion with its bank cleaved, and a histological examination revealed a diffuse, small cell (Lymphoma Study Group classification) malignant lymphoma. An immunohistochemical analysis of the surgical specimen was positive for LCA/CD45, UCLH-1/CD45RO, and Leu-4/CD3, and negative for L-26/CD20, and it was diagnosed to be primary gastric T-cell malignant lymphoma.