Background and Aims The systemic immune inflammation index (SII) is considered to be a new inflammatory and prognostic mark, but its relationship with the prognosis of gastric cancer patients is still controversial. Therefore, this study was conducted to evaluate the association between SII and the prognosis of gastric cancer patients through systematic review and Meta-analysis, so as to provide evidence-based medical information for clinical decision-making.Methods The cohort studies on the relationship between SII and the prognosis of gastric cancer patients were collected by searching PubMed, EMBASE, web of science and Cochrane library databases. The retrieval time was from the database inception of to July 28, 2020. After the literature screening, data extraction and assessment of bias risk of the included studies by two independent researchers, the data were analyzed by Stata 12.0 software.Results A total of 12 retrospective cohort studies were included, including 7 244 patients. Meta-analysis showed that the overall survival (OS) and disease-free survival (DFS)/recurrence free survival (RFS) were shortened in gastric cancer patients with higher SII value (HR=1.28, 95% CI=1.16-1.41, P<0.001; HR=1.34, 95% CI=1.06-1.70, P=0.013). Subgroup analyses stratified by country, treatment method and sample size all showed that higher SII value was associated with shorter OS (all P<0.05). When the SII value reached or exceeded the cut-off value of 600, the higher SII was associated with shortened OS (HR=1.56, 95% CI=1.34-1.80, P<0.001), but when the SII value was lower than cut-off value of 600, there is no significant association between SII and OS (P>0.05). When the study time was ≥ 6 years, the higher SII value was associated with the shorter OS (HR=1.65, 95% CI=1.21-2.25, P<0.001), but there was no significant association between SII and OS when the study time was less than 6 years (P>0.05). In addition, patients with higher SII value were associated with advanced TNM stage (OR=2.45, 95% CI=1.75-3.44, P<0.001), increased risk of lymph node metastasis (OR=1.72, 95% CI=1.27-2.32, P<0.001), large tumor size (OR=2.45, 95% CI=1.75-3.44, P<0.001), and poor differentiation (OR=2.45，95% CI=1.75-3.44，P<0.001).Conclusion SII value can be used as a prognostic marker for gastric cancer patients, and those with relatively high SII value may face an unfavorable prognosis. However, the above conclusion still needs to be verified by more high-quality studies, due to the limitations in the number and quality of the included studies.