A 55-year-old woman with Familial Adenomatous Polyposis and prior proctocolectomy underwent surveillance endoscopy, which identified a 20 mm polypoid lesion in the superior duodenal angle near the minor papilla. NBI chromoendoscopy revealed enlarged crypts and an irregular pattern suggestive of high-grade dysplasia. Endoscopic mucosal resection was performed after submucosal injection of diluted epinephrine, followed by diathermic snare resection. Active oozing bleeding occurred but was effectively controlled with an over-the-scope clip, achieving immediate hemostasis. The patient experienced only mild post-procedural abdominal pain and no adverse events such as perforation or delayed bleeding. Histopathology showed a tubular adenoma with focal high-grade dysplasia and free pedicle margin. Annual surveillance was recommended.