Faced with limited and inadequate treatment options for patients with advanced gastric cancer or gastroesophageal junction cancer (GC/GEJC), researchers have turned toward, with the support of promising clinical trials, anti-PD-1/anti-PD-L1 antibody therapy. But there are also different clinical trial results. To better assess its efficacy and safety, we integrated data from 13 eligible studies for a systematic review and meta-analysis. To comprehensively evaluate the efficacy and safety of anti-PD-1/anti-PD-L1 antibody therapy in the treatment of advanced GC/GEJC patients. PubMed, Web of Science, Cochrane Library ,and EMBASE databases were searched to identify eligible articles with outcomes including objective response rate (ORR), disease control rate (DCR), overall survival (OS), progression-free survival (PFS), and adverse events (AEs) of anti-PD-1/anti-PD-L1 antibody therapy. Our study encompassed a total of 13 trials totaling 1618 patients. The outcomes showed a pooled ORR and DCR of 15% (95% confidence interval [CI]: 14%-18%) and 40% (95%CI: 33%-46%), respectively. The pooled 6-mo OS and PFS were 54% (95%CI: 45%-64%) and 26% (95%CI: 20%-32%), respectively, and the 12-mo OS and PFS were 42% (95%CI: 21%-62%) and 11% (95%CI: 8%-13%), respectively. In addition, the incidence of any-grade AEs and grade ≥ 3 AEs was 64% (95%CI: 54%-73%) and 18% (95%CI: 16%-20%), respectively. Most importantly, PD-L1 positive patients exhibited a higher ORR rate than PD-L1 negative patients (odds ratio = 2.54, 95%CI: 1.56-4.15). Anti-PD-1/anti-PD-L1 antibody therapy has shown promising anti-tumor efficacy with manageable AEs in advanced GC/GEJC patients, with PD-L1 overexpressing patients exhibiting a higher ORR. What is more, the clinical efficacy of anti-PD-1/PD-L1 combined with traditional chemotherapy drugs is even better, although the occurrence of AEs still causes considerate concerns. ©The Author(s) 2020. Published by Baishideng Publishing Group Inc. All rights reserved.