A 68-year-old Japanese woman underwent radiotherapy for gastric lymphoma. Although lymphangiectasia was sparsely observed in the second portion of the duodenum before radiotherapy, the number of pinpoint white spots obviously increased after the treatment. Although the duodenal lymphangiectasia gradually progressed, the patient had no features of protein-losing enteropathy. This case highlights the importance of endoscopic observation of the duodenum after irradiation to the abdomen as radiotherapy may secondarily cause intestinal lymphangiectasia.