Objective To investigate the correlation between plasma vitamin D levels and a novel inflammatory marker， the systemic immune-inflammatory index （SII）， in patients with type 2 diabetes. Methods This study adopted a cross-sectional design， in which patients diagnosed with type 2 diabetes who were admitted to the First Affiliated Hospital of Guangxi Medical University were enrolled as study participants. Data on demographic characteristics， medical history， physical examination findings， and laboratory test results were systematically collected. Participants were categorized into three groups based on their serum vitamin D levels： deficient， insufficient， and sufficient. The relationship between vitamin D levels and the SII was evaluated using a multivariate linear regression model. Additionally， a restricted cubic spline model was employed to assess the nonlinear dose-response association between vitamin D levels and SII. Results This study enrolled a total of 5，716 patients with type 2 diabetes. A statistically significant difference in the SII was observed across groups with varying vitamin D levels （P < 0.05）， with the highest SII value found in the vitamin D-deficient group. Multivariate linear regression analysis revealed that， after adjusting for potential confounding factors including gender， age， season of blood collection， body mass index， hypertension， dyslipidemia， and chronic kidney disease， vitamin D levels were negatively associated with SII （β = -2.68， 95%CI： -3.56 to -1.81， P < 0.001）. Compared with the vitamin D-deficient group， the vitamin D-sufficient group exhibited significantly lower SII levels （β = -78.42， 95%CI： -137.90 to -18.93， P = 0.01）. Furthermore， the restricted cubic spline model indicated a nonlinear dose-response relationship between vitamin D levels and SII （P < 0.001）. Conclusion There is a significant inverse correlation between plasma vitamin D levels and the SII in patients with type 2 diabetes.