Primary malignant melanoma arising from the eustachian tube is extremely rare. We report the case of a 63-year-old white man who presented with a 1-month history of left-sided hearing loss and aural fullness. Flexible fiberoptic laryngoscopy detected a blue-purple mass that appeared to arise from the left lateral nasopharynx. Computed tomography demonstrated an enhancing mass arising from an orifice of the left eustachian tube. The tumor was debulked endoscopically and was confirmed to have originated in the left eustachian tube. Histologically, the tumor was made up of heavily pigmented pleomorphic spindle cells with frequent mitoses. The tumor cells were immunohistochemically positive for S-100 protein, HMB-45, Melan-A, and PNL-2. The final diagnosis was a mucosal malignant melanoma. We also performed a nested polymerase chain reaction assay for several genes of interest, including CTLA-4, IL-17A, IL-17B, IL-17C, IL-17D, IL-17E, IL-17F, PLZF, Foxp3, RORγt, CD27, and CD70. These genes have been studied mainly in cutaneous melanomas, especially for the development of immunotherapy, but only very limited studies have been done on mucosal melanomas. Our investigation found upregulation of CTLA-4, IL-17A, IL-17C, and IL-17E. Based on our finding of CTLA-4 upregulation, it may be suggested that our patient might have had low antitumor immunity and that he might have benefited from CTLA-4 blockade. On the other hand, upregulation of IL-17A and IL-17E might reflect increased antitumor immunity, which could suggest that patients with a mucosal melanoma might benefit from immunomodulators associated with the effect of Th17. These genes also have great potential to help melanoma patients obtain tailored treatment, and they can be used as biomarkers for predicting prognosis.