Purpose <p>Older persons frequently experience postoperative acute pain and delirium, however, there is little clinical data on their possible interrelationships.</p> Methods <p>A secondary analysis of multicenter prospective data, including tertiary hospitals in Five Chinese Regions, from April 2020 to April 2022. Older persons (≥ 65&#xa0;years) scheduled for elective abdominal surgery under general anesthesia, without auditory, visual, or cognitive impairments, who completed assessments for postoperative delirium, anxiety, and depression. The impact of early postoperative acute pain on postoperative delirium in older persons undergoing abdominal surgery was evaluated using univariate and multivariate logistic regression, propensity score matching, and subgroup analysis.</p> Results <p>The study cohort included 2,674 patients. The median age was 70&#xa0;years, and 66.5% of the patients were male. Delirium occurred in 13.2% (<i>n</i> = 354) of patients in postoperative 7&#xa0;days (POD 7), with a significantly higher prevalence in the moderate-to-severe pain group (unadjusted: 16.3 vs. 9.6%, <i>p</i> &lt; 0.001). Moderate-to-severe pain on POD1 showed a significant association with delirium occurrence in POD 7 according to univariate logistic regression (OR 1.83, 95% CI 1.445–2.319, <i>p</i> &lt; 0.001), multivariable logistic regression (OR 1.63, 95% CI 1.274–2.084, <i>p</i> &lt; 0.001), and propensity score matching (PSM) model (OR 1.44, 95% CI 1.108–1.879, <i>p</i> = 0.006).</p> Conclusion <p>In older persons undergoing elective abdominal surgery, moderate-to-severe pain on POD 1 was significantly associated with both delirium and depressive symptoms within 7&#xa0;days of surgery. Improved early postoperative analgesic strategies may aid in preventing delirium.</p> <p><i>Trial registration</i>: ClinicalTrials.gov identifier: NCT06964893 (2025-05-07).</p>