Synchronous occurrence of malignant lymphoma and carcinoma, both located in the intestinal tract, is unusual. We report a unique case of an adenocarcinoma of the cecum and a simultaneous mantle cell lymphoma of the colon, terminal ileum, and regional lymph nodes in an 85-year-old man. Grossly, the adenocarcinoma was identified as a cecal mass. Lymphomatous involvement of the gastrointestinal tract was evident only on microscopic examination. The terminal ileum and colon showed microscopic disseminated multiple mucosal nodules, with involvement of the regional lymph nodes. There was no involvement of distant organs, suggesting that the mantle cell lymphoma was early in its evolution without formation of polyps or a mass lesion. To our knowledge, this is the fourth reported case with this association and the second case that showed early involvement of the gastrointestinal tract with mantle cell lymphoma without polyp formation.