This report presents a case of collision tumors of low-grade B-cell lymphoma and adenocarcinoma in the sigmoid colon of an 81-year-old man. All surgically resected regional mesenteric lymph nodes were found to be occupied by low-grade B-cell lymphoma, and one lymph node showed the presence of adenocarcinoma. Low-grade B-cell lymphoma was also observed in the resected spleen. Moreover, concurrent tuberculosis infection in the resected colon was proven by the presence of positive results obtained with polymerase chain reaction analysis of the mycobacterial DNA. Systemic chemotherapy was administered for advanced colon cancer with lung metastasis, and anti-tuberculosis treatment was also prescribed. The occurrence of synchronous lymphoma and adenocarcinoma of the colorectal region is rare. Furthermore, collisions of these different entities are also extremely unusual. The accurate clinical determination of the dominant tumor and a timely follow-up are required for the proper treatment of these cases.