Background and aims: Oral mucositis (OM) is common and distressing toxicity in children on chemotherapy. There is limited number of safe and effective therapeutic options available for OM. Ketamine oral rinse has shown promising results in few studies in adults. This randomized, double-blind placebo-controlled trial aimed to test the efficacy of ketamine mouthwash in reducing chemotherapy-induced severe OM pain in children. Methods: Children aged 8-18 years with severe OM were randomized to a single dose of ketamine mouthwash (4 mg/ml solution; dose 1 mg/kg) or a placebo. A sample size of 44 patients was determined. Pain score (6-point faces scale) was noted at baseline and 15, 30, 45, 60, 120, 180, and 240 min. The outcome variables were a reduction in pain score, need for rescue medications, and adverse events. Results: The baseline characteristics were comparable in the two groups. The mean OM pain at 60 min decreased by 1.64 points (CI 1.13-2.14) in the ketamine group and 1.32 points (CI 0.76-1.87) in the placebo group (p=0.425), with a group difference of 0.32 points. Rescue pain medication (at 60 min) was required in 13.6% in the ketamine group and 18.2% in the placebo group (p=1.000). There were no significant adverse events observed. Conclusions: Among children on cancer chemotherapy with severe OM, ketamine mouthwash at a dose of 1 mg/kg did not significantly reduce OM pain. It did not decrease the need for rescue pain medications. Further research is warranted to test higher doses of ketamine for a clinically significant effect.