Background and objective: The Systemic Immune Inflammation Index (SII), a non-invasive bi-omarker utilizing platelet, neutrophil, and lymphocyte counts, serves as an objective and pre-dictive indicator for the comprehensive assessment of host immune status and inflammation in Non-Small Cell Lung Cancer (NSCLC) patients. This study aims to evaluate the prognostic value of SII in advanced-stage NSCLC patients. Methods: This analytical observational study with a retrospective cohort design included 65 advanced-stage NSCLC patients who received a mini-mum of three cycles of chemotherapy from January 2020 to December 2022 at Dr. M Djamil Hospital Padang. Survival duration was defined as the period (in months) from the initiation of therapy until the patient either deceased or completed the observation phase, with the last fol-low-up in December 2023. Survival was categorized into two groups: less than one year and equal to or exceeding one year. The optimal SII cut-off values for one-year survival were deter-mined using receiver operating characteristic (ROC) curve analysis. Results: The average SII in the group with less than one-year survival was higher compared to the other group (3,414.95 x 109/L vs 1,517.95 x 9/L). The ROC analysis revealed an Area Under the Curve (AUC) value of 0.745 (95% CI 0.62-0.87). The ideal SII cutoff for predicting one-year survival was 1760.00 x 109/L, with a sensitivity of 74% and specificity of 81%. Conclusion: The study's robust findings demonstrate that higher SII values are linked to shorter survival in advanced-stage NSCLC patients. The SII serves as a promising prognostic