We describe the case of a 78-year-old man with collision tumor from the primary malignant lymphoma and adenocarcinoma in the ascending colon. He suffered anemia from sigmoid colon cancer, and colonoscopy revealed early-stage colorectal cancer with a diameter of 20 mm in the cecum, the biopsy specimen showed moderately differentiated adenocarcinoma. Contrast-enhanced computed tomography (CT) revealed bowel wall thickening with contrast enhancement at the cecum; however, no lymph node and organ metastases were found. As above, we performed laparoscopic ileocecal resection with D3 lymph node dissection. The postoperative course was uneventful, and he was discharged from the hospital on postoperative day 11. Histopathological findings were moderately differentiated adenocarcinoma which invaded the muscularis propria and serosa from the submucosa, while the adjacent serosa showed a highly diffuse proliferation of atypical cells with an irregular nuclear-to-cytoplasmic ratio. Besides, immunohistochemical staining findings were diffuse large B-cell lymphoma, and diffuse large B-cell lymphoma was coexistent with moderately differentiated adenocarcinoma. We treated the patient with cyclophosphamide, doxorubicin, vincristine, and prednisolone in combination with rituximab (R-CHOP therapy) during 3 months postoperatively. When the 8 courses had been completed, postoperative positron emission tomography-CT (PET-CT) confirmed complete response, and the disease control has been doing well. Malignant lymphoma of the colorectal region is relative rare, and the occurrence of synchronous lymphoma and adenocarcinoma of the colon is also rare. Furthermore, collision tumor by these different entities is very unusual. We presented here such a case. The accurate clinical determination of the dominant tumor and a close follow-up is required for proper treatment in these cases.