Background Endoscopic submucosal dissection (ESD) was recently developed to allow en bloc resection of early neoplasia of the GI tract, including colorectal neoplasia. The endoscopic technique is technically demanding and not yet standardized, and new devices are needed. Objective This study aimed to evaluate the efficacy and safety of a new device that combines the functions of injection and cutting. Design Prospective, pilot, single-arm study. Methods Consecutive patients with rectal laterally spreading tumors (LSTs) 3 cm or larger unsuitable for en bloc resection were enrolled. ESD was performed with a new device that allows cutting and coagulation as well as a needleless, tissue-selective mucosal and submucosal elevation through an axial water-jet channel. Main Outcome Measurement The primary endpoint of the study was the en bloc resection rate achieved with ESD in a Western hospital setting. Results Overall, ESD was attempted in 40 consecutive patients (27 male, mean age 65.3 years) with rectal LSTs larger than 3 cm (72.5% LSTs, nongranular type, 5% depressed type, 22.5% protruding type). The mean lesion size was 46.8 ± 10.9 mm (range 33-80 mm). The mean procedure time was 86.1 ± 35.5 minutes (range 40-190 minutes). The en bloc resection rate was 90% (36/40). In the remaining patients, resection was completed with a piecemeal approach. The rate of curative resection (R0) was 32 of 40 LSTs (80%). Two patients with submucosal invasion were referred for surgery. Perforation occurred in 1 patient (2.5%), which was managed conservatively. Postoperative bleeding occurred in 2 patients (5%) and was treated by endoscopic hemostasis. Limitations Single-center study with a relatively small number of patients. Conclusions ESD is a safe and effective method to provide en bloc and curative resection of large rectal LSTs. The operating time and adverse event rate were comparable to those of previously published data from Japanese experts.