Primary mucosa-associated with a lymphoid tissue (MALT) lymphoma is a rare distinct subtype of non-Hodgkin’s lymphoma that occurs in approximately 8% of all non-Hodgkin lymphomas. Primary gastrointestinal MALT lymphoma usually occurs in the stomach, but duodenal involvement is extremely rare. Therefore, the clinical manifestations, treatment, and prognosis of primary duodenal MALT lymphoma have not yet been validated because of its rarity. This paper reports a case of a 40-year-old male with primary duodenal MALT lymphoma who was treated successfully with radiation therapy alone. A 40-year-old male visited for a medical check-up. Esophagogastroduodenoscopy revealed whitish multi-nodular mucosal lesions in the second and third portions of the duodenum. Biopsy specimens from mucosal lesions in the duodenum were reported to be suspicious for MALT lymphoma of the duodenum. He received a total dose of 3,000 cGy in 15 fractions with external beam radiation therapy for three weeks. Three months after radiation therapy, an endoscopic examination revealed complete resolution of the duodenal lesions. The follow-up 12 months after radiation therapy showed no evidence of tumor recurrence. (Korean J Gastroenterol 2023;81:129-132)