This study explored the relationship between the pretreatment systemic immune-inflammation index (SII) and overall survival (OS) in gastric cancer (GC) patients. A systemic literature search was performed to find out the articles that estimated the relationship of SII with specific clinical parameters and OS in GC patients. Nine articles (including 10 studies) were included. A total of 3,850 cases were eventually included. In GC patients, there was no association between pretreatment SII and gender (OR=0.991, p=0.944) or differentiation (OR=1.093, p=0.687). However, pretreatment SII was related to depth of tumor invasion (OR=0.340, p <0.001), lymph node metastasis (OR=0.447, p <0.001) and TNM stage (OR=0.361, p <0.001) in GC patients. The ORs of 1-year, 3-year and 5-year OS were 0.467 (I2=0.0%; p=0.682), 0.355 (I2=85.6%; p <0.001) and 0.507 (I2=56.4%; p=0.057). The pretreatment SII could be used as an indicator of the depth of tumor invasion, lymph node metastasis, TNM stage and overall of gastric cancer patients. However, more multi-centres researches are needed to confirm these findings. Key Words: Systemic immune-inflammation index (SII), Prognosis, Gastric cancer.