Background: Malnutrition before major abdominal surgery is a prevalent concern among patients. At Gia Dinh People's Hospital, we introduced the Controlling Nutritional Status (CONUT) score for preoperative malnutrition risk assessment. Our objective was to investigate the prevalence of preoperative malnutrition identified by the CONUT score among major abdominal surgical patients and its impact on their surgical outcomes. Methods: In this prospective cohort study, we analyzed 100 patients scheduled for major abdominal surgery between December 2021 and April 2022. Our primary outcome was the prevalence of malnutrition determined by the CONUT score, while the secondary outcome was the impact of malnutrition on surgical outcomes. Results: The CONUT score identified 52% of the 100 patients as malnourished. Postoperative complications occurred in 10% of the patients. The CONUT score showed predictive capability for postoperative complications, with an area under the curve (AUC) of 0.734. Conclusions: Our findings highlight the significant prevalence of preoperative malnutrition risk among patients undergoing major abdominal surgery. Furthermore, they emphasize the heightened risk of postoperative complications among patients with malnutrition.