AIMS: Symptomatic treatment is insufficient for chemotherapy- or targeted therapy-induced oral mucositis (OM) pain, and benzydamine mouthwash is not commercially available in Japan. We evaluated the analgesic effects of an in-hospital preparation of 0.25% indomethacin spray (IMS) on anticancer drug-induced OM pain. METHODS: This single-arm prospective trial enrolled 20 patients (median age 62.0 years) with OM and numerical rating scale scores of ≥5 who were undergoing chemotherapy or targeted therapy in our hospital. Pain scores were recorded using a visual analog scale (VAS) before and 30 min after IMS administration. Pain relief (PR) scores were recorded at 15, 30, and 60 min after IMS administration; total PR after 60 min (TOTPAR60 ) was calculated, and the mean PR score after 3 days (PR3days ) was determined. RESULTS: The median (interquartile range) OM grade of the participants was 2.0 (2.0-2.3). The VAS score decreased significantly at 30 min after IMS administration (p = .001). The median (interquartile range) TOTPAR60 and PR3days were 6.0 (3.8-7.3) and 2.0 (2.0-3.0), respectively. CONCLUSIONS: IMS helped improve patients' quality of life. The risk of systemic adverse effects was low because of the low dose administered. IMS effectively relieved anticancer drug-induced OM pain and may be useful for immediate self-medication.