We herein present a quite atypical case of primary gastrointestinal mucosa-associated lymphoid tissue (MALT) lymphoma in the transverse colon. Computed tomography and endoscopic ultrasonography revealed diffuse thickening of the wall, and colonoscopy showed a white-colored mucosa with reduced superficial vessels in the entire transverse colon. The lesion was diagnosed as MALT lymphoma by pathological examination of the biopsied specimen. Secondary macroglobulinemia of IgM-kappa type was also found in the present case. After chemotherapy and radiation, the lesions in the transverse colon improved and the patient has been in good condition without any evidence of recurrence for more than 1 year.