OBJECTIVE: Systemic immune-inflammation index (SII) has been reported in numerous studies to effectively predict the survival outcomes of urinary system cancers; however no agreement has been reached. This meta-analysis aimed to explore the prognostic significance of pre-treatment SII in tumours of the urinary system. METHODS: Relevant published articles were selected from Web of Science, PubMed, Embase, and the Cochrane Library up to 30 August 2020. The hazard ratios (HRs) with 95% confidence intervals (CIs) were computed to estimate the associations of pre-treatment SII with overall survival (OS), progression-free survival (PFS), cancer-specific survival (CSS) in urinary system cancers. RESULTS: 13 papers were included in our meta-analysis. From the combined data, we found that a high pre-treatment SII indicated a markedly worse OS (HR = 1.98; 95% CI: 1.75-2.23;  < .001), PFS (HR: 2.08; 95% CI: 1.32-3.26;  = .002), and CSS (HR: 2.41, 95% CI: 1.73-3.35,  < .001). Additionally, patients with an elevated SII value might have undesirable pathological characteristics, including a large tumour size, a poor differentiation grade, and an advanced tumour stage (all  < .001). CONCLUSIONS: Pre-treatment SII could be used as a non-invasive and promising biomarker to indicate the prognosis of urinary system cancer patients.KEY MESSAGES:This meta-analysis evaluates the predictive value of systemic immune-inflammation index (SII) for patients with urinary system cancer.A high pre-treatment SII indicates a poor prognosis.SII can serve as a promising non-invasive biomarker to help clinicians assess the prognosis and develop treatment strategies for urinary system cancer patients.