Background: Activation and clonal expansion of tumor antigen-specific T cells in cancer patients are crucial for immunotherapies. Recent reports indicate that CD39 is a marker to distinguish tumor-reactive CD8 T cell populations from bystander populations. In gastric cancer (GC), anti-CD39 antibody, anti-PD-1 antibody, and FOLFOX combination therapy may be effective therapy at AACR 2022. Methods: We retrospectively reviewed the data of 62 GC patients who underwent gastrectomy at our hospital (2017 - 2018) and examined their rejected specimen’s immune-microenvironment using immunohistochemistry. Then we prospectively enrolled 16 GC patients. Their tumor tissue and tumor-infiltrating lymphocytes (TILs) were extracted from surgical specimens, these fractions were subjected to the RNA-seq analysis (n = 3), CD107a degranulation assay (n = 7) and 3H-Thymidine incorporation assay (n = 7). Results: The densities of CD39⁺CD8 TILs positively correlated with the densities of CD4 T cells and B-cells in GC tissues (P = 0.01, < 0.01, respectively). Furthermore, higher densities of CD39⁺CD8 TILs in GC patients were associated with better overall survival (P< 0.01). We classified TILs into four groups based on expression levels of the PD-1 and CD39. The proportion of CD39⁺CD8 T cells was significantly higher in GC tissues than their PBMCs (n = 13) and non-cancerous tissues (n = 4). CD39⁺PD-1⁺CD8 (DP) TILs in primary tumors were elevated expression of multiple inhibitory receptors such as LAG3, TIGIT and reduced expression of IL-2 by RNA-seq analysis. T cells expanded in vitro from DP-TILs (n = 5) efficiently recognized autologous tumor cells in CD107a assay compared with other populations, and considered to be tumor-reactive T cells fraction. However, since these DP TILs in primary tumors show exhausted phenotypes, capacities to kill tumor are expected to be reduced in the tumor-microenvironment (TME). By adding ATP to the DP-TILs in vitro at the concentration reported in the TME, we observed reduction of cell proliferation by 3H-thymidene incorporation assay and of degranulation by CD107a assay, and we observed recovery of these changes by adding anti-CD39 inhibitory antibody. Conclusions: DP-TILs may have superior T cell receptors recognizing tumor-antigens and contribute to tumor eradication, especially when administrated anti-CD39 antibody.