OBJECTIVES: There is concern that underwater endoscopic mucosal resection (UWEMR) uses buoyancy to elevate the lesion for snare resection, resulting in a shallower resection depth than that in endoscopic mucosal resection (EMR). We aimed to compare conventional EMR and UWEMR in terms of resection depth. METHODS: We retrospectively reviewed cases in which EMR or UWEMR was performed for superficial non-ampullary duodenal epithelial tumors of ≤20 mm between April 2018 and February 2024. The endpoints were histological complete resection rate, en bloc resection rate, presence of muscularis mucosa and submucosa in the resection specimen, and submucosal index calculated from the resection specimen. RESULTS: EMR was performed on 19 lesions and UWEMR was performed on 52 lesions. Histological complete and en bloc resection rates were not significantly different between EMR and UWEMR (57.9% and 63.5%, respectively, CONCLUSION: UWEMR potentially includes the submucosa within the specimen.