BACKGROUND: Postoperative delirium (POD) is a common complication that may occur from 24 to 72 h after cardiac surgery. Frailty is a chronic syndrome that leads to a decline in physiological reserve and to disability. The associations between frailty and POD are unclear. AIMS: To investigate associations between POD and frailty in patients undergoing cardiac surgery and to analyse predictors of POD. METHODS AND RESULTS: Convenience sampling was used to recruit 152 patients who underwent cardiac surgery in two medical centres in northern Taiwan. Preoperative frailty in these patients was evaluated using Fried's frailty phenotype. Delirium in patients was assessed from postoperative day 1 to day 5 using the confusion assessment method for intensive care units. A total of 152 patients who underwent cardiac surgery included 68 (44.74%) prefrail patients and 21 (13.81%) patients with frailty after the surgery. Ten patients (6.58%) developed delirium after cardiac surgery. The occurrence of delirium peaked at postoperative day 2, and the average duration of delirium was 3 days. A case-control comparison revealed a significant correlation between preoperative frailty and POD. Significant predictors of POD in patients undergoing cardiac surgery included the European System for Cardiac Operative Risk Evaluation II, preoperative arrhythmia, and preoperative anxiety and depression. CONCLUSION: Preoperative frailty was correlated with POD. Preoperative arrhythmia, anxiety, and depression are predictors of POD. Nurses should perform preoperative assessments of surgical risk and physiological and psychological conditions of patients undergoing cardiac surgery and monitor the occurrence of POD.