A 33-year-old woman had hematochezia for 5 months. A total colonoscopy showed an easily-bleeding elevated lesion with a nodular lesion in the lower rectum. Histological findings of a conventional biopsy from the lesion showed MALT (mucosa-associated lymphoid tissue) lymphoma and she was diagnosed as stage I. The monoclonality of B-cells was not detected by polymerase chain reaction products for immunoglobulin heavy chain. We selected antibiotic therapy because Helicobacter pylori was detected in culture of the patient's gastric biopsy specimens. The antibiotic therapy was successful, but her symptoms worsened. We therefore gave her with 30 Gy radiation therapy. She recovered a month after the radiation. Histological complete remission was confirmed 4 months after the radiation. Adverse events of the radiation therapy included anal pain for 1 month and premature ovarian failure. Radiation therapy may be useful for localized rectal MALT lymphoma.