ABSTRACT BACKGROUND: A common site of neuroendocrine tumors (NETs) is the rectum. The technique most often used is endoscopic mucosal resection with saline injection. However, deep margins are often difficult to obtain because submucosal invasion is common. Underwater endoscopic mucosal resection (UEMR) is a technique in which the bowel lumen is filled with water rather than air, precluding the need for submucosal lifting. OBJECTIVE: This study aimed to evaluate the efficacy and safety of UEMR for removing small rectal neuroendocrine tumors (rNETs). METHODS: Retrospective study with patients who underwent UEMR in two centers. UEMR was performed using a standard colonoscope. No submucosal injection was performed. Board-certified pathologists conducted histopathologic assessment. RESULTS: UEMR for small rNET was performed on 11 patients (nine female) with a mean age of 55.8 years and 11 lesions (mean size 7 mm, range 3-12 mm). There were 9 (81%) patients with G1 rNET and two patients with G2, and all tumors invaded the submucosa with only one restricted to the mucosa. None case showed vascular or perineural invasion. All lesions were removed en bloc. Nine (81%) resections had free margins. Two patients had deep margin involvement; one had negative biopsies via endoscopic surveillance, and the other was lost to follow-up. No perforations or delayed bleeding occurred. CONCLUSION: UEMR appeared to be an effective and safe alternative for treatment of small rNETs without adverse events and with high en bloc and R0 resection rates. Further prospective studies are needed to compare available endoscopic interventions and to elucidate the most appropriate endoscopic technique for resection of rNETs.