Background <p>Systemic immune-inflammation index (SII) and Prognostic nutritional index (PNI) are emerging inflammation-related indicators that have previously been linked to prognosis of cancer patients, but the results have been inconsistent. This meta-analysis aims to review and report the latest data regarding the prognostic value of the SII and PNI in cancer patients treated with immune checkpoint inhibitors (ICIs).</p> Methods <p>We searched the relevant literature in PubMed, Cochrane Library, and Embase databases from inception to February 15, 2026. Meta-analyses were performed for overall survival (OS) and progression-free survival (PFS) to assess the prognostic role of SII and PNI in cancer patients treated with ICIs. The pooled OS and PFS were estimated using hazard ratios (HR) with 95% confidence intervals (95% CI) by the random-effects model. Subgroup analysis was also conducted to investigate the correlation between indicators and study features.</p> Results <p>A total of 27 studies comprising 4000 patients treated with ICIs were included in this meta-analysis. In terms of the results based on multivariate analysis, the pooled results revealed that high SII level was an unfavorable predictor for OS (HR = 2.42, 95% CI = 1.88–3.10) and PFS (HR = 1.99, 95% CI = 1.36–2.90). In addition, lower level of PNI was correlated with worse OS (HR = 2.28, 95% CI = 1.68–3.08), as well as PFS (HR = 1.84, 95% CI = 1.56–2.17). Furthermore, subgroup analysis was conducted, and we found similar conclusions.</p> Conclusion <p>Our findings support SII and PNI as promising biomarkers for adequately studied cancer types, but extending them to all malignancies requires prospective validation in less-studied cancers.</p>