A 61-year-old patient with a large duodenal laterally spreading tumor (LST-G 6.5 ×4 cm) below the papilla of Vater. Literature reports superficial non-ampullary duodenal tumor (SNADT) prevalence at 1.0–1.5% in retrospective series and 4.6% prospectively, with adenomas at 0.03–0.4%.In collaboration with a visiting expert, enbloc underwater endoscopic submucosal dissection (ESD) was accomplished without complications.Histopathology confirmed R0 resection of a tubular adenoma with low-grade dysplasia and focally high-grade dysplasia.The water immersion technique enhanced visualization, endoscope stability, and mucosal-submucosal separation, reducing risk of perforation in this delicate area.Our center's treatment alternative,Whipple surgery,highlights ESD as a safe,minimally invasive option for selected large duodenal polyps by skilled expert.