BACKGROUND: There are few data on the clinical implications of immunosuppressive protein expression in tumors and immune cell infiltration within the tumor microenvironment in patients with gastric cancer (GC). METHODS: In this study, 243 patients with curatively resected GC were included. The levels of immunosuppressive protein expression [programmed cell death 1 ligand 1 (PD-L1), cytotoxic T-lymphocyte antigen 4 (CTLA-4), and indoleamine 2,3-dioxygenase (IDO)] in tumors and the densities of immune cells [CD3(+), CD4(+), CD8(+), or PD-1(+) cells] within the tumor microenvironment were measured using immunohistochemical analysis. RESULTS: Positive PD-L1, CTLA-4, and IDO expression was observed in 43.6, 65.8, and 47.7% of the patients, respectively. Expression of PD-L1, CTLA-4, and IDO was related to less advanced stage, intestinal type, and well/moderately differentiated adenocarcinoma (P < 0.05). PD-L1 expression was related to better disease-free survival (DFS) and overall survival (OS) in GC [PD-L1(+) vs. PD-L1(-) tumors: 5-year DFS rate, 82.6 vs. 66.9%; 5-year OS rate, 83.0 vs. 69.1% (P values <0.05)]. Survival outcomes were also better in patients with a higher density of CD3(+) cells within the tumor microenvironment than in those with a lower density of CD3(+) cells [5-year DFS rate, 80.9 vs. 67.0%; 5-year OS rate, 82.5 vs. 68.0% (P values <0.05)]. In multivariate analysis, these two immune markers had a prognostic impact on survival, independent of other clinical variables. CONCLUSIONS: GC patients with immunosuppressive protein expression (PD-L1, CTLA-4, or IDO) had distinct clinicopathological characteristics. PD-L1(+) expression and a high-CD3 tumor microenvironment are favorable prognostic markers in GC.