Purpose <p>Malnutrition is a prevalent and detrimental complication in patients with nasopharyngeal carcinoma (NPC) undergoing radiotherapy. Early identification and intervention are crucial for optimizing outcomes.</p> Methods <p>We retrospectively analyzed data from 383 patients with NPC who underwent radiotherapy. We employed logistic regression analysis to identify risk factors for malnutrition and developed a nomogram for its prediction. The nomogram was internally and externally validated using bootstrap methods and calibration curves.</p> Results <p>Body mass index (BMI), chemotherapy cycles, central venous catheter (CVC), and alanine aminotransferase (ALT) were identified as independent risk factors for malnutrition. The nomogram demonstrated good discriminatory ability (AUC: 0.786 in the training set, 0.687 in the validation set) and clinical utility, with high net benefit in decision curve analysis.</p> Conclusions <p>This nomogram offers a practical tool for predicting nutritional risk in patients with NPC undergoing radiotherapy. Its application can facilitate early identification of at-risk patients and guide targeted interventions to improve clinical outcomes.</p>