Objectives: To evaluate the efficacy of hypochlorous acid (HOCl) nasal spray as an adjuvant therapy after functional endoscopic sinus surgery (FESS). Material and methods: Patients with chronic rhinosinusitis who had received FESS for treatment were recruited and assigned to one of two groups at random at one month post-surgery. In the HOCl group, patients received 0.02% HOCl nasal spray three times a day for two months. In the control group, normal saline (NS) nasal irrigation was given. Before FESS and before and after nasal spray or irrigation, patients completed the Taiwanese version of the 22-item Sino-Nasal Outcome Test (TWSNOT-22). In addition, patients received endoscopic examination, acoustic rhinometry, smell test, saccharine transit test, and bacterial cultures obtained from their middle meatus. Results: Seventy-eight patients completed the study. Among them, 41 received HOCl nasal spray, and 37 received NS irrigation. Endoscopic score significantly decreased after 2-month HOCl nasal spray (p = 0.036). TWSNOT-22 score also decreased, although insignificantly (p = 0.285). In contrast, TWSNOT-22 score significantly decreased after NS nasal irrigation (p = 0.017), but endoscopic score did not significantly decrease (p = 0.142). Conclusions: Our results showed that HOCl nasal spray had a similar effect to that of NS nasal irrigation in postFESS care. It can be an alternative of NS nasal irrigation for its convenient application.