A 51-year-old male patient was found to have a 20-mm submucosal tumor at the descending part of the duodenum on upper gastrointestinal endoscopy, as part of a general health examination. We diagnosed this as a somatostatin-producing, neuroendocrine tumor (NET). Contrast-enhanced abdominal computed tomography confirmed early-enhanced wall thickening at the descending part of the duodenum and a 25-mm area of lymphadenopathy near the horizontal part of the duodenum. We performed a subtotal stomach-preserving pancreaticoduodenectomy. Histopathological diagnosis primarily localized the tumor to the submucosa, with partial infiltration of the subserosa. We also found island-shaped and tubular growth of cells with oval-shaped nuclei, and somatostatin was positive by immunostaining. There was 1 mitotic figure/10 HPF, and the Ki-67 labeling index was 1.42% ; thus, we diagnosed a G1 somatostatin-producing NET. Metastasis was observed in #13 and #14d lymph nodes in accordance with General Rules for the Study of Biliary Tract Cancer (6th Edition). Duodenal somatostatin-producing NET is relatively rare. Thus, we report this case with a literature review.