BACKGROUND: Several surgical procedures for posterior nasal neurectomy have been reported, but no conclusion has been reached about which procedure is best. OBJECTIVES: The aim is to evaluate the improvement in symptom medication scores for resection of the posterior nasal nerve trunk in an underwater environment, with submucous inferior turbinectomy, without injuring the sphenopalatine artery (SPA) in severe allergic rhinitis. MATERIAL AND METHODS: Improvements in symptom medication scores were retrospectively compared between 27 consecutive cases who underwent resection of the posterior nasal nerve trunk with turbinoplasty in an underwater environment without injuring the SPA (Underwater group) and, as a historical control, 16 consecutive cases who underwent resection of peripheral branches of the posterior nasal nerve with turbinoplasty (Control group). RESULTS: The improvements in symptom medication scores in the Underwater group were significantly better than in the Control group (3.07 vs. 1.96, p = 0.02). CONCLUSIONS AND SIGNIFICANCE: By using underwater posterior nasal neurectomy, we can easily and safely resect the posterior nasal nerve trunk under a clear surgical view without injuring the SPA. This technique with submucous inferior turbinectomy may, more than resection of peripheral branches of the posterior nasal nerve, be able to reduce the medication score and symptom medication score.