In this study we assessed the clinical significance of an epithelial-mesenchymal transition (EMT) gene signature and explored its association with the tumor microenvironment related to immunotherapy in patients with head and neck squamous cell carcinoma (HNSCC). Genes were selected when mRNA levels were positively or negatively correlated with at least one well-known EMT marker. We developed an EMT gene signature consisting of 82 genes. The patients were classified into epithelial or mesenchymal subgroups according to EMT signature. The clinical significance of the EMT signature was validated in three independent cohorts and its association with several immunotherapy-related signatures was investigated. The mesenchymal subgroup showed worse prognosis than the epithelial subgroup, and significantly elevated PD-1, PD-L1, and CTLA-4 levels, and increased interferon-gamma, cytolytic, T cell infiltration, overall immune infiltration, and immune signature scores. The relationship between PD-L1 expression and EMT status in HNSCC after treatment with TGF-β was validated in vitro. In conclusion, the EMT gene signature was associated with prognosis in HNSCC. Additionally, our results suggest that EMT is related to immune activity of the tumor microenvironment with elevated immune checkpoint molecules.