The systemic immune-inflammation index (SII) is a novel biomarker that reflects the balance between the host immune response and inflammation, two key pathophysiological processes involved in sepsis. This meta-analysis aimed to evaluate the relationship between SII at admission and short-term mortality in patients with sepsis. Literature searches were performed in PubMed, Embase, Web of Science, CNKI, and Wanfang up to August 30, 2024, using relevant search terms. Observational studies that reported the association between SII and short-term mortality in sepsis patients were included. Risk ratios (RRs) and 95% confidence intervals (CIs) comparing the incidence of mortality within 90 days in patients with sepsis with a high versus low SII were calculated. Nine cohort studies, with a total of 25,626 patients, were included. A high SII at admission was significantly associated with an increased risk of all-cause short-term mortality in sepsis patients (RR: 1.51, 95% CI: 1.31-1.67, P < 0.001), with moderate heterogeneity (I² = 43%). Sensitivity analyses confirmed the robustness of these findings. Subgroup analyses suggested a stronger association in patients younger than 67 years compared to those aged 67 years or older (P = 0.04), but no significant differences were observed based on sex, SII cutoff values, or follow-up duration. In conclusion, this meta-analysis demonstrates that elevated SII at admission is associated with an increased risk of short-term mortality in sepsis patients, particularly in younger individuals. Further research is needed to validate these findings and explore their clinical implications.