Objective <p>Obese patients with type 2 diabetes mellitus (T2DM) face unique long-term nutritional risks following Roux-en-Y gastric bypass (RYGB) surgery. This study aims to evaluate postoperative nutritional status in this population, identify key preoperative predictive factors, and develop and validate a clinical scoring system for predicting the risk of postoperative malnutrition.</p> Methods <p>This single-center retrospective cohort study consecutively included obese T2DM patients (body mass index [BMI] ≥ 32.5&#xa0;kg/m²) who underwent RYGB between January 2020 and January 2023. The primary outcome was a composite endpoint of malnutrition at 12 months postoperatively, defined as moderate anemia (hemoglobin &lt; 100&#xa0;g/L) or hypoalbuminemia (albumin &lt; 30&#xa0;g/L). Univariate and multivariate logistic regression analyses were performed to identify independent predictors. Discrimination of the nomogram-based predictive model was assessed by the area under the receiver operating characteristic curve (AUC). Calibration was evaluated using bootstrap calibration plots.</p> Results <p>A total of 195 patients were included in the final analysis, with a 12-month postoperative malnutrition incidence of 31.8% (62/195). Female gender (odds ratio [OR] = 2.702, 95% confidence interval [CI]: 1.034–7.058) and diabetes duration ≥ 5 years (OR = 4.890, 95% CI: 1.416–16.894) were independent risk factors for postoperative malnutrition, whereas higher preoperative levels of albumin, hemoglobin, ferritin, and 25-hydroxyvitamin D were protective factors. The joint predictive model incorporating these six factors demonstrated excellent discriminative ability for predicting postoperative malnutrition and good calibration.</p> Conclusion <p>The preoperative clinical risk prediction model based on six readily available indicators shows robust predictive performance, guiding individualized nutritional management.</p>