BACKGROUND: This cohort study aimed to assess the association between atherogenic index of plasma (AIP) and the risk of delirium in intensive care unit (ICU) patients. METHODS: Using the Medical Information Mart for Intensive Care IV (MIMIC-IV) database, we analyzed 2,104 adult patients with ICU stays of ≥48 h. The relationship between AIP and delirium risk was investigated using logistic regression and restricted cubic splines (RCS). Subgroup analyses and interaction tests were conducted to assess effect modification. The primary endpoint was the incidence of delirium, with results expressed as odds ratio (OR) and 95% confidence intervals (CI). RESULTS: After adjusting for confounding variables, elevated AIP was significantly associated with delirium risk (OR 1.48, 95% CI 1.15-1.91, p = 0.003). RCS analysis revealed a J-shaped nonlinear relationship (P for nonlinearity = 0.01) between AIP and delirium with an inflection point at AIP = 0.33. Subgroup analyses confirmed consistent associations across demographic and clinical strata (all P for interaction > 0.05). CONCLUSION: This study suggests that AIP may serve as an independent biomarker for delirium risk in ICU patients, indicating its potential utility for early identification of high-risk patients. The nonlinear association highlights the need for personalized risk assessment based on AIP values.