We present a case series of underwater microvascular decompression (MVD) for hemifacial spasm (HFS) and an evaluation of its feasibility and safety. This retrospective study was conducted at a single institution and included 20 patients with HFS who underwent underwater MVD between September 2019 and January 2021. Surgery was performed in 3 steps, as follows: exoscopic wound opening (soft tissue, bone, dura, and arachnoid around the cerebellomedullary cistern), underwater endoscopic surgery (decompression of the facial nerve), and exoscopic wound closure. In underwater endoscopic surgery, the surgical field was continuously irrigated with artificial cerebrospinal fluid. Abnormal muscle response and brainstem auditory evoked potentials (BAEPs) were monitored. Neurovascular conflicts were clearly observed in all patients without fogging and soiling of the endoscope lens. HFS was completely relieved in 19 patients (95%). An amplitude reduction of wave V of BAEPs of more than 50% was not observed in any of the cases. In 5 cases (25%), the latency of wave V of BAEPs was prolonged for more than 1.0 ms; these changes completely or near completely returned to baseline values at dural closure in all 5 cases. A postoperative complication of transient facial palsy was observed in 1 patient (5%) during postoperative days 10-30. There were no other complications. Our findings suggest that underwater MVD is a safe and feasible option for the treatment of HFS. However, it did not show advantages over conventional endoscopic MVD when the protective effect on the eighth cranial nerve was evaluated.