Patients undergoing cardiovascular surgery often experience postoperative complications and Activities of Daily Living (ADL) decline, even in the absence of preoperative malnutrition. Since postoperative ADL decline is particularly serious in elderly patients, this study aimed to identify a nutritional assessment tool that is effective in predicting postoperative ADL decline. Patients undergoing open-heart surgery with cardiopulmonary bypass were assessed using eight nutritional assessment tools, including the Malnutrition Universal Screening Tool (MUST), the Global Leadership Initiative on Malnutrition (GLIM) criteria, the Nutritional Risk Screening 2002 (NRS-2002), the Subjective Global Assessment (SGA), the Controlling Nutritional Status (CONUT) score, the Prognostic Nutritional Index (PNI), the Geriatric Nutritional Risk Index (GNRI), and the Mini Nutritional Assessment-Short Form (MNA-SF). A total of 197 patients were enrolled in this study, with a mean age of 70.4 ± 11.6 years old; 31.0% were female. Depending on the nutritional assessment tool, 17.8% to 78.2% of patients were identified as at risk of malnutrition. Among the various assessment tools, the MUST was the most effective in predicting postoperative ADL decline (OR 4.75, 95% CI 1.37-16.5, = 0.014) and was also associated with severe complications and length of hospital stay (OR 3.08, 95% CI 0.20-0.76, = 0.009). On the other hand, nutritional risk detected by assessment tools other than MUST, including MNA-SF and GLIM, could predict postoperative complications but showed no relationship to ADL decline. We concluded that MUST was the most useful preoperative nutritional assessment tool for predicting outcomes, particularly for assessing the risk of postoperative ADL decline in patients undergoing cardiovascular surgery.