Purpose: Systemic immune-inflammation index (SII) has been reported in numerous studies to effectively predict the pathological features and survival outcomes of urinary system cancers, but no agreement has been reached. The aim of this meta-analysis is to explore the prognostic significance of pretreatment 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 August 30, 2020. The pooled hazard ratios (HRs), odds ratios (ORs), and standard mean differences (SMDs) with 95% confidence intervals (CIs) were computed to estimate the associations of pretreatment SII with overall survival (OS), progression-free survival (PFS), cancer-specific survival (CSS), and clinicopathological parameters in urinary system cancers.Results: Overall, a total of 13 papers published from 2016 to 2020 involving 15 datasets comprising 3974 patients were finally included in our meta-analysis. From the combined data, we found that high SII prior to treatment indicated markedly worse OS (HR =1.98, 95% CI: 1.75–2.23; p < 0.001), PFS (HR: 2.08, 95% CI: 1.32–3.26, p = 0.002) with high heterogeneity (I2 = 80.8%, p < 0.001), and CSS (HR: 2.41, 95% CI: 1.73–3.35, p < 0.001). In addition, patients who have an elevated SII value before receiving treatment might have undesirable pathological characteristics, including large tumour size, poor differentiation grade, and advanced tumour stage (all p < 0.001).Conclusion: Pretreatment SII could be used as a noninvasive and promising biomarker for indicating the prognosis of patients with urinary system cancers.