Purpose Targeting of the PD-1/PD-L1 signalling pathway is a promising treatment strategy in several cancers. The aim of this study was to assess the expression of PD-L1 on tumour cells and tumour-infiltrating lymphocytes (TILs) in gastric cancer (GC) and its prognostic impact. Materials and Methods A total of 240 patients who were diagnosed with GC at Sun Yat-sen University Cancer Centre (SYSUCC) from May 2008 to December 2013 were included in this study. PD-L1 expression was detected by immunohistochemistry (IHC) in all GC tumour specimens. The Cox proportional hazard regression model was used to assess the association between PD-L1 expression and overall survival (OS). Results The positive rates of PD-L1 expression on tumour cells and TILs were 74.8% and 65.8%, respectively. Patients with poor tumour differentiation had higher positive rates of PD-L1 expression on tumour cells (p=0.023). There was no significant association between PD-L1 expression on tumour cells and other clinicopathological data. In TILs, PD-L1 expression was significantly higher in patients who underwent surgery (p=0.031) and were in the late stage (p=0.021) than those without surgery and in the early stage. Patients with positive PD-L1 expression on TILs had a significantly shorter five-year OS than those with negative PD-L1 expression (14.2 vs 18.3; p=0.001); therefore, PD-L1 expression on TILs is an independent prognostic factor. However, PD-L1 expression on tumour cells is not associated with OS (p=0.945). Conclusion Our findings suggest that PD-L1 expression on TILs may be a predictive factor for immunotherapy of PD-1/PD-L1 pathway inhibitors.