Background: Systemic immune-inflammation index (SII) provides convincing evaluation of systemic immune and inflammatory condition in human body. Its correlation with prostate cancer (PCa) risk remains uncharted. The principal objective of this investigation was to elucidate the association between SII and the risk for PCa in middle-aged and elderly males., Materials and Methods: Analysis entailed multivariate linear and logistic regression, generalized additive model, and smoothing curve fitting using resource from 2007 to 2010 National Health and Nutrition Examination Survey (NHANES). To ascertain robustness and consistency of this association across different demographic strata, we conducted rigorous subgroup analyses and interaction tests., Results: Among 3359 participants, those with elevated SII displayed higher total prostate-specific antigen (tPSA) levels, higher risk for PCa, and lower free/total PSA (f/t PSA) ratio. Specifically, each unit increase of log 2 (SII) was associated with a 0.22 ng/mL increase in tPSA (β: 0.22, 95% confidence intervals [CI] 0.05-0.38), a 2.22% decline in f/t PSA ratio (β: -2.22, 95% CI -3.20 to -1.23), and a 52% increased odds of being at high risk for PCa (odds ratio [OR]: 1.52, 95% CI 1.13-2.04). People in the top quartile of log 2 (SII) exhibited 0.55 ng/mL increased tPSA (β: 0.55, 95% CI 0.19-0.90), 4.39% reduced f/t PSA ratio (β: -4.39, 95% CI -6.50 to -2.27), and 168% increased odds of being at high risk for PCa (OR: 2.68, 95% CI 1.32-5.46) compared to those in the bottom quartile., Conclusion: Systemic immune and inflammatory condition, as represented by SII, is independently and positively associated with tPSA levels and the risk for PCa, as well as independently and negatively associated with f/t PSA ratio among middle-aged and older US males. These findings may enhance the effectiveness of PCa screening in predicting positive biopsy results., (© 2024 The Author(s). Immunity, Inflammation and Disease published by John Wiley & Sons Ltd.)