We report here on a case of colonic extranodal marginal zone B-cell lymphoma of the mucosa-associated lymphoid tissue (MALT) in a 17-year-old male with a history of selective IgA deficiency. The colonoscopic findings showed prominent lymphoid hyperplasia along the terminal ileum and multiple ulceroinfiltrative nodular lesions at the transverse colon and rectum. Immunohistochemically, an extensive infiltration by a CD20-positive lymphoid cell population that displayed a prominent lymphoepithelial structure was detected. Multiple hypermetabolic lymph nodes were found in the cervical, axillary, mediastinal and abdominal regions, in addition to the colon, by performing PET-CT. After administering rituximab weekly for 4 weeks, he had significant regression of the colonic lesions and the lesions in multiple lymph nodes. He remains progression free 10 months after finishing the treatment. This case report demonstrates the potential efficacy of rituximab as a single therapeutic agent for patients with colonic extranodal marginal zone B-cell lymphoma, and even for a patient with an extensive disease status.