Objective: The aim of this study was to evaluate the feasibility and safety of transcanal underwater endoscopic bone resection (TUEBR) of the external auditory canal (EAC) for the management of cholesteatoma involving the antrum and mastoid. Study Design: Retrospective case review. Setting: Tertiary referral center. Patients: Pediatric and adult patients with primary cholesteatoma extending to the antrum and mastoid who underwent transcanal endoscopic ear surgery (TEES) with TUEBR between March 2016 and June 2017. Intervention: A rigid 2.7 mm diameter, 18 cm length Hopkins-rod telescope with an endoscopic sheath was inserted in the EAC and continuously perfused with saline during the dissection. TUEBR was performed to expose extensive cholesteatoma by using a high speed drill with curved burrs and a protected shaft. Next, removal of visible disease, reconstruction of the resected EAC, ossiculoplasty, and tympanoplasty were accomplished with TEES. Results: There were no intra- or postoperative severe complications such as facial palsy and inner ear injury except one patient suffering from secondary labyrinthitis. There was a negative linear relationship (r = -0.909) between the procedure time and procedure number of TUEBR. There was a weak relationship (r = 0.224) between the procedure time of TUEBR and the degree of the extension of cholesteatoma into the antrum and mastoid. There were two cases with residual cholesteatoma at 12 and 22 months follow-up postoperatively. Conclusion: TUEBR is a safe and efficient technique for the resection of EAC bone and transcanal exposure of extensive cholesteatoma that would otherwise require mastoid dissection.