Background and aims <p>We aimed to assess the efficacy of prophylactic mucosal defect closure in reducing delayed adverse events (AEs) after endoscopic resection (ER) of large-to-giant laterally spreading superficial non-ampullary duodenal epithelial tumors (SNADETs).</p> Materials and methods <p>We performed a retrospective review of 219 patients with 230 large-to-giant laterally spreading SNADETs who underwent ER at Zhongshan Hospital, Fudan University. The preoperative baseline information, procedure-related AEs and follow-up outcomes were compared between lesions with complete closure and those with incomplete closure or no closure.</p> Results <p>The mean maximal diameter was 22.8 ± 13.8&#xa0;mm (mean ± SD), with 67 lesions (29.1%) ≥ 30&#xa0;mm. A total of 42 lesions (18.3%) had an occupied circumference ≥ 50%. There were 32 lesions (13.9%) with incomplete closure or non-closure of the mucosal defect. A total of 18 lesions (7.8%) experienced delayed AEs, including delayed perforation in 6 cases (2.6%) and delayed bleeding in 12 cases (5.2%). Lesions with incomplete closure or non-closure had a higher incidence of delayed AEs than lesions with complete closure (34.4% vs 3.5%, <i>P</i> &lt; 0.001). Multivariable regression analyses revealed that incomplete or no mucosal defect closure (OR = 5.154; 95% CI = 1.443–18.405; <i>P</i> = 0.012) and lesions located on the anal side of the major papilla (OR = 11.457; 95% CI = 1.404–93.499; <i>P</i> = 0.023) were independent risk factors for delayed AEs. Lesions with an occupied circumference ≥ 50% (OR = 3.171; 95% CI = 1.024–9.818; <i>P</i> = 0.045) and irregular shape (OR = 5.557; 95% CI = 1.226–25.184; <i>P</i> = 0.026) were independent risk factors for incomplete closure or non-closure.</p> Conclusions <p>Prophylactic mucosal defect closure is an effective technique for reducing delayed AEs after ER of large-to-giant laterally spreading SNADETs.</p> Graphical abstract <p></p>