This study investigates the association between the systemic immune-inflammation index (SII) and psoriasis, a chronic inflammatory disease. The research utilized data from the National Health and Nutrition Examination Survey (NHANES) for the years 2009-2014, involving 17,913 participants. SII, calculated as the ratio of platelet count to lymphocyte count, was found to have a positive correlation with psoriasis, with the risk of developing psoriasis increasing as SII tertiles increased. This association was significant in women, individuals with a BMI of ≥30 kg/m², and non-stroke and non-cancer subjects. Subgroup analyses revealed two consecutive inverted U-shaped patterns in the relationship between SII and psoriasis, with the most significant inflection point at an SII value of 797.067. The study suggests that elevated SII levels are associated with an increased risk of psoriasis, but further large-scale prospective studies are needed to validate these findings.This study investigates the association between the systemic immune-inflammation index (SII) and psoriasis, a chronic inflammatory disease. The research utilized data from the National Health and Nutrition Examination Survey (NHANES) for the years 2009-2014, involving 17,913 participants. SII, calculated as the ratio of platelet count to lymphocyte count, was found to have a positive correlation with psoriasis, with the risk of developing psoriasis increasing as SII tertiles increased. This association was significant in women, individuals with a BMI of ≥30 kg/m², and non-stroke and non-cancer subjects. Subgroup analyses revealed two consecutive inverted U-shaped patterns in the relationship between SII and psoriasis, with the most significant inflection point at an SII value of 797.067. The study suggests that elevated SII levels are associated with an increased risk of psoriasis, but further large-scale prospective studies are needed to validate these findings.