Malnutrition is highly prevalent among patients with gastrointestinal (GI) cancers and has a significant effect on surgical outcomes. Despite the growing recognition of this relationship, standardized nutritional assessment and intervention protocols remain inconsistent across healthcare settings. A prospective cohort study was carried out over a one-year period (June 2023–June 2024) to determine the associations between preoperative nutritional status and postoperative outcomes in patients undergoing major resection for GI malignancies. Consecutive patients who underwent elective major GI cancer resection were enrolled. The study utilized validated tools to assess nutritional status, including serum albumin (ALB), body mass index (BMI), and the Patient-Generated Subjective Global Assessment (PG-SGA). The principal outcomes were major postoperative complications (Clavien–Dindo grade ≥3), length of hospital stay, and thirty-day mortality. Among 294 patients, 128 (43.5%) were identified as nutritionally at risk preoperatively. Patients classified as malnourished experienced significantly higher 30-day mortality, with a rate of 12.5% versus 4.2% in well-nourished patients (p = 0.008). They also had a greater incidence of major postoperative complications, occurring in 35.9% of cases versus 18.1% among their well-nourished counterparts (p < 0.001). In addition, the median length of hospital stay was notably longer in malnourished individuals (14 days) than in those with adequate nutritional status (9 days; p < 0.001). Multivariate analysis revealed severe malnutrition as an independent risk factor for adverse postoperative outcomes, with an odds ratio (OR) of 2.84, a 95% confidence interval (CI) ranging from 1.52 – 5.31, and a p value of 0.001. These findings demonstrate that preoperative malnutrition significantly increases perioperative morbidity and mortality among patients undergoing GI cancer surgery. These findings advocate the integration of routine nutritional screening and tailored interventions into the perioperative care pathway.