Background: Conversion surgery after chemotherapy responds to unresectable gastric cancer has long-term prognosis, however, chemotherapy after surgery should also be considered to improve treatment outcome. The effectiveness of immune checkpoint inhibitors for gastric cancer has been proved. In this study, we investigated the association between PDL-1 expression and CD8⁺ T cell infiltration in the primary tumor and prognosis of patients who underwent conversion surgery after S-1-based multidrug chemotherapy in unresectable gastric cancer. Methods: We evaluated PDL-1 expression and CD8 T cell infiltration by immunohistochemical staining and examined the relationship with prognosis in 90 patients with Stage IV gastric cancer who underwent gastrectomy in our department from 2007 to 2015. Results: 90 cases included 47 cases of palliative resection, 26 cases of CY1P 0cases and 17 cases of conversion surgery. Among Conversion Surgery cases, 11 (65%) patients underwent R0 surgery, and the 5-year survival rate was 66%. The 5-year survival rate of non-curative resected cases was 16%. In immunohistochemical examination, PDL-1 high expression was a poor prognostic factor in palliative surgery and CY1P0 cases. Meanwhile, in the case of Conversion Surgery, the 5-year survival rate was 58%, 32%, 61%, and 30% in the order of PDL-1 high expression group, low expression group, CD8 T cell high infiltration, and low infiltration group, respectively. Prognosis of PDL-1 high expression group and CD8 T cell infiltration group was good. There was no association between curative resection and neither of CD8 T cell infiltration or PDL-1 expression. Conclusions: We found that the prognosis was relatively good in patients where local immune response was enhanced, regardless of R0 excision. Our results suggested that infiltration of CD8 T cells in the primary tumor may be a prognostic predictor, and it is expected to use an immune check point inhibitor after conversion surgery for unresectable gastric cancer.