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Lung squamous cell carcinoma (LUSC) is a common subtype of lung cancer. Th1 cells contribute to antitumor immune responses. However, there are few studies on Th1 cells in LUSC. CD8+ T cells are the main driver of the antitumor immunity, targeting tumor cells killing. Th1 cells play an important auxiliary role in this process. Here, we used single‐cell RNA‐seq (scRNA‐seq) to analyze qualified CD4+ T cells and Th1 cells (defined CD4+ T cells with 1 or more of STAT1+, STAT4+, T‐bet+, and IFN‐γ+ as Th1 cells) from tissues of 8 LUSC patients. Then, we validated Th1 cells and CD8+ T cells of 32 LUSC patients by multiplex immunofluorescence staining and immunohistochemistry. Finally, we used flow cytometry to detect IFN‐γ of CD4+ T cells in human PBMCs coincubated with LUSC‐derived supernatant to simulate a tumor inhibitory microenvironment. ScRNA‐seq showed IFN‐γ+ Th1 cells account for 25.28% of all Th1 cells. Gene ontology and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analyses of differentially expressed genes between IFN‐γ+ Th1 cells and IFN‐γ– Th1 cells confirmed the decreased IFN‐γ is associated with endoplasmic reticulum stress (ER stress). Multiplex immunofluorescence staining and immunohistochemistry proved there was a positive correlation between IFN‐γ+ STAT1+ T‐bet+ Th1 cells and CD8+ T cells. Flow cytometry showed IFN‐γ secreted by Th1 cells is decreased. These findings support the claim that Th1 cells' function is suppressed in LUSC. Through scRNA‐seq, we found that the decreased Th1 cells' function is associated with ER stress, which requires further study. Overall, these findings may produce a new method for the treatment of LUSC. [ABSTRACT FROM AUTHOR]