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Abstract Background Carbon‐ion radiotherapy (C‐ion RT) is effective for head and neck mucosal melanoma (HN‐MM), including radioresistant mucosal melanoma. Melanoma also responds effectively to immune checkpoint inhibitors (ICIs). Data on the efficacy and safety of ICIs for HN‐MM are insufficient. Aims To analyze the efficacy and safety of ICI salvage therapy in patients with HN‐MM recurrence after C‐ion RT. Methods and Results This retrospective study analyzed the medical records of 52 patients with HN‐MM treated with C‐ion RT between 2012 and 2020. A dose of 57.6 or 64.0 Gy (relative biological effectiveness) was provided in 16 fractions. The primary endpoint was 3‐year overall survival (OS) rate. The median follow‐up time was 26.8 months for all patients. A total of 29 patients had local recurrence or distant metastasis, and 16 patients who received ICI therapy. The 3‐year OS rate in the ICI group (n = 16) and best supportive care group (n = 13) were 53.8% and 0.0%, respectively (p = 0.837); the difference was not statistically significant. There were no deaths after 1 year among patients who underwent ICI therapy. No adverse events associated with C‐ion RT were related to or exacerbated by ICI. Conclusion ICI salvage therapy is effective and safe for patients with HN‐MM recurrence after C‐ion RT.