Oral mucositis (OM) pain due to anticancer chemo- and radiotherapy has a very negative impact on patient quality of life. However, no high-quality studies have been performed regarding the analgesic efficacy of indomethacin (IM) oral spray for OM pain. This randomized, placebo-controlled, double-blind trial aimed to evaluate the analgesic efficacy of IM oral spray for OM pain due to anticancer chemo- and radiotherapy. From July 2015 to December 2016, we enrolled adult cancer patients with OM pain that was due to anticancer chemo- or radiotherapy and was rated 4 or higher on Brief Pain Inventory (BPI) Item 5. Patients were randomly assigned in a 1:1 ratio to receive either IM oral spray or placebo. The primary endpoint was the change in the BPI Item 6 ("current pain") score from before to 30 minutes after treatment. Secondary endpoints were the areas under the curves of BPI Item 6 at 15, 60, 120, 180, and 240 minutes after treatment; five items related to meals and conversation from the European Organization for Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire, Head and Neck Module 35; the Clinical Global Impressions-Improvement (CGI-I) scale; and adverse events. A total of 60 patients were assigned to receive IM oral spray (n = 33) or placebo spray (n = 27). The average change in the BPI item 6 score from before to 30 minutes after treatment was -1.85 (95% confidence interval: -2.37 to -1.32) in the IM spray group and -0.59 (-1.02 to -0.16) in the placebo group, indicating a significant difference (-1.26, -1.94 to -0.57, P < 0.01). The pain improvement persisted for 180 minutes. The intergroup differences in ability to drink liquids, ease in conversing, and CGI-I were all significant (P = 0.03, P = 0.02, and P < 0.01, respectively). No serious adverse events were reported. IM oral spray alleviated short-term OM pain due to anticancer chemo- and radiotherapy, and may reduce the difficulty in eating meals. Copyright © 2021 American Academy of Hospice and Palliative Medicine. Published by Elsevier Inc. All rights reserved.