A 67-year-old woman with bloody stools underwent esophagogastroduodenoscopy and colonoscopy, which revealed the presence of a submucosal tumor in the stomach and an adenocarcinoma in the ascending colon. Preoperative screening disclosed an additional 10-cm tumor in the abdomen between the gastric and colonic lesions. Single-balloon enteroscopy was therefore performed. A nonstenotic, circumferential, bleeding ulcerative lesion was found in the jejunum, and the biopsy revealed malignant lymphoma (ML). A partial resection of the small intestine, ascending colectomy, and wedge resection of the stomach were performed, then systemic chemotherapy for ML was started 2 weeks after surgery. Triple gastrointestinal malignant tumors with different histologies are extremely rare, and have not been previously reported. Single-balloon enteroscopy in this case led to a definite diagnosis by biopsy, thus allowing the patient to receive chemotherapy as soon as possible after surgery.