Postoperative delirium (POD) is a frequent neuropsychiatric disorder characterized by sudden onset, particularly in elderly patients. The aim of this study is to assess the relationship between cognitive function, depression status, and the risk of POD in geriatric patients undergoing major abdominal surgery. This prospective cohort study included 100 patients aged 65 years and older who underwent major abdominal surgery. Preoperatively and postoperatively, patients underwent cognitive assessments using the mini-mental state examination (MMSE), geriatric depression scale (GDS), three-word recall test, and clock drawing test. Demographic data and test results were subsequently analyzed statistically. The mean age of the participants was 70.8 ± 5.9 years. POD occurred in 18% of patients. Statistically significant differences were observed between preoperative and postoperative values for MMSE, GDS, and clock drawing test (P < .001). Postoperative GDS results showed a significantly lower rate of normal results in the delirium group (16.7%) compared to the non-delirium group (52.4%) (P = .021). Each 1-unit decrease in preoperative MMSE score was associated with an approximately 10-fold increase in the risk of delirium (OR: 9.7; 95% CI: 1.3-71.4). In conclusion, major abdominal surgery can increase depression and impair cognitive function in elderly individuals. POD is a common complication in the elderly, with significant health consequences. The MMSE may be a useful tool for predicting the risk of postoperative delirium.