Current research indicate that inflammation is linked to the development of overactive bladder (OAB). The aim of this study was to examine the correlation between OAB and the systemic immunity-inflammation index (SII) in the USA. We analyzed data from 31,881 participants in the National Health and Nutrition Examination Survey 2005-2018. SII, calculated as platelet count × neutrophil count/lymphocyte count, was categorized into quartiles. OAB was defined by the presence of urge urinary incontinence and nocturia. Weighted logistic regression models were used to examine the independent relationship between SII and OAB, adjusting for demographic factors, kidney function, and diabetes status. The results showed that each tenfold increase in log-transformed SII was associated with an 18% higher odds of OAB (OR 1.18, 95% CI 1.08-1.28) in the fully adjusted model. Compared to the lowest SII quartile, the highest quartile had a 28% increased OAB risk (OR 1.28, 95% CI 1.12-1.47). The positive association between SII and OAB risk was consistently observed across subgroups stratified by age, sex, race, marital status, education, and poverty level. Our study reveals a positive correlation between SII levels and OAB, indicating that higher SII levels are associated with an increased likelihood of developing OAB.