Tonsillectomy surgery is associated with severe postoperative pain that usually requires analgesics including opioids. Pain control is still a big problem after tonsillectomy surgery. We aimed to evaluate the efficacy of preemptive analgesia using montelukast for pediatric post-tonsillectomy pain management. This is the first-time use of montelukast in post-tonsillectomy pain. Double-blind, controlled-randomized study. University teaching and research hospital. A total of 60 children, aged 5 to 15 years, American Society of Anesthesiologist class I-II, scheduled for elective tonsillectomy were enrolled in this clinical trial study. The patients were randomized into 2 groups: the montelukast group (group M, n = 30) and control group (group C, n = 30). Group M recieved an oral montelukast tablet and group C recieved placebo at 2400pm on the morning before surgery. Post-tonsillectomy pain was evaluated with the Wong-Baker FACES Scale during the 24 hours after surgery. Patients' intraoperative hemodynamic parameters and intraoperative and postoperative complications were recorded. There were statistically significant differences between group C and group M for Wong-Baker FACES pain rating scale scores (P < .05). In the 24 hours after surgery, the total number of patients using rescue analgesics was higher in group C than in group M, and the difference was statistically significant (P < .001). There was no significant difference in demographic parameters (P > .05). There were no significant differences in postoperative nausea and vomiting, otalgia, trismus, fever, or halitosis between the groups (P > .05). Preemptive montelukast can be used safely to reduce the serious pain caused by tonsillectomy in children.