CD4 T cell helper and regulatory function in human cancers has been well characterised. However, the definition of tumor-infiltrating CD4 T cell exhaustion and how it contributes to the immune response and disease progression in human gastric cancer (GC) remain largely unknown. A total of 128 GC patients were enrolled in the study. The expression of CD39 and PD-1 on CD4 T cells in the different samples was analysed by flow cytometry. GC-infiltrating CD4 T cell subpopulations based on CD39 expression were phenotypically and functionally assessed. The role of CD39 in the immune response of GC-infiltrating T cells was investigated by inhibiting CD39 enzymatic activity. In comparison with CD4 T cells from the non-tumor tissues, significantly more GC-infiltrating CD4 T cells expressed CD39. Most GC-infiltrating CD39 CD4 T cells exhibited CD45RA CCR7 effector-memory phenotype expressing more exhaustion-associated inhibitory molecules and transcription factors and produced less TNF-α, IFN-γ and cytolytic molecules than their CD39 CD4 counterparts. Moreover, inhibition of CD39 enzymatic activity enhanced their functional potential reflected by TNF-α and IFN-γ production. Finally, increased percentages of GC-infiltrating CD39 CD4 T cells were positively associated with disease progression and patients' poorer overall survival. Our study demonstrates that CD39 expression defines GC-infiltrating CD4 T cell exhaustion and their immunosuppressive function. Targeting CD39 may be a promising therapeutic strategy for treating GC patients.