The association of frailty with postoperative delirium has not been fully investigated in patients undergoing cardiovascular surgery. Therefore, this study aimed to investigate whether preoperative hand grip strength is associated with postoperative delirium. This retrospective study included patients aged >65 years who had undergone elective cardiovascular surgery using cardiopulmonary bypass at a Japanese university hospital between April 2020 and February 2022. We defined low hand grip strength as hand grip values of <275 n and <177 n for men and women, respectively. Postoperative delirium was assessed using the confusion assessment method during patients’ intensive care unit stay. The odds ratio of low hand grip strength for postoperative delirium was estimated using multiple logistic analysis, which was adjusted for prominent clinical factors. Ninety-five patients with a median age of 74 years were included in the final analysis, and 31.5% of them had low hand grip strength. Postoperative delirium occurred in 37% of patients, and the odds ratio of low preoperative hand grip strength for postoperative delirium was 4.58 (95% confidence interval: 1.57–13.2). Thirty-seven patients experienced postoperative delirium after cardiovascular surgery using cardiopulmonary bypass, and low preoperative hand grip strength was positively associated with its occurrence.