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Background: Although anti-HER2 therapies have been widely used against gastric carcinoma, the prognostic significance of HER2 overexpression remains unclear. Previous studies failed to provide convincible evidence due to inconsistent HER2 evaluation criteria and heterogeneous clinical characteristics. Objectives: To figure out the prognostic significance of HER2 expression in gastric cancer, we rigorously designed and conducted this study. Design: Meta-analysis. Data sources and methods: Record retrieval was performed by searching PubMed, Web of Science, Cochrane Library, Embase, ASCO, and ESMO meeting libraries from inception to November 2022. Cohort studies investigating overall survival comparison between HER2-positive and HER2-negative gastric cancer patients were included. Both resectable and advanced cases were separately collected while HER2 evaluation standards should be consistent across eligible studies. Newcastle–Ottawa Scale was used for quality assessment. Overall survival was the only endpoint and effect size was presented by hazard ratio (HR) with its 95% confidence interval. The pooled calculation was conducted on Review Manager 5.4. Results: Thirty studies were eligible, including 9945 patients. Eligible studies were mostly high quality (n = 31). Regarding resectable cases (n = 22), HER2-positive groups had significantly worse prognosis than HER2-negative counterparts (HR 1.56, 95%CI 1.32–1.85, p < 0.00001). For HER2-positive patients with advanced gastric cancer (n = 10), HER2 overexpression was also an unfavorable survival indicator (HR 1.70, 95%CI 1.23–2.35, p = 0.001). Potential heterogeneous studies had been eliminated while outcomes remained stable by sensitivity analysis. Subgroup analysis suggested HER2-positive patients had a poorer prognosis in both East Asian (resectable: HR 1.56; advanced: HR 1.32) and non-East Asian countries (HR 1.58; HR 3.27). Conclusion: As a novel survival biomarker in gastric cancer, HER2 overexpression indicates unfavorable prognosis among both resectable and advanced patients, irrespective of East Asian or non-East Asian populations. Trial registration: PROSPERO (CRD42020168051). [ABSTRACT FROM AUTHOR]