INTRODUCTION: Pembrolizumab has been approved for the first-line treatment of patients with advanced gastric cancer (GC) and gastroesophageal junction (GEJ) cancer. However, the results of several clinical trials are not entirely consistent, and the dominant population of first-line immunotherapy for advanced GC/GEJ still needs to be precisely determined. PURPOSE: The aim of this meta-analysis was to assess the efficacy and safety of pembrolizumab in the treatment of advanced GC/GEJ. METHODS: We conducted computerized searches across multiple databases, including PubMed, Cochrane Library, Web of Science, and Embase. We established the inclusion criteria to comprise randomized clinical trials examining the efficacy of pembrolizumab in late-stage GC/GCJ cancer. We conducted a meta-analysis of outcome measures using STATA 14.0 software. RESULTS: A total of six studies involving 1,448 cases were included in this analysis. The results of the meta-analysis indicate that, when compared to chemotherapy, patients in the pembrolizumab group experienced a significant reduction in the risk of mortality in terms of overall survival (OS) (hazard ratio [HR] = 0.72, 95% confidence interval [CI]: 0.65-0.79, p <
0.01). In terms of progression-free survival (PFS), pembrolizumab was associated with a similar PFS as compared to chemotherapy (HR = 0.88, 95% CI: 0.73-1.07, p = 0.206). Subgroup analyses based on PD-L1 expression levels indicated a significantly longer PFS with pembrolizumab in subgroups of patients with PD-L1 CPS ≥10 but not in those with PD-L1 CPS ≥1 and PD-L1 CPS ≥5. Subgroup analyses based on distinct geographical regions revealed a comparable effect of PFS in patients residing in Asia or the USA Subgroup analysis based on tumor sites consistently demonstrated a similar effect of PFS in patients with EC/GEJ tumors and GC patients. CONCLUSION: Our findings demonstrated that pembrolizumab led to a significant extension in OS and objective response rate, along with a favorable tolerability profile compared to chemotherapy. Furthermore, the observed survival benefits were particularly pronounced in subgroup patients with a CPS of ≥10. Given the potential limitations inherent in our study, it is imperative to underscore the necessity for further large-scale RCTs to corroborate our results.