Studies regarding malnutrition in patients with nasopharyngeal carcinoma (NPC) using the Global Leadership Initiative in Malnutrition (GLIM) criteria are still limited. Our study aimed to investigate the prevalence of malnutrition using the GLIM criteria in NPC patients receiving radiotherapy and explore the relationship between pre-radiotherapy (pre-RT) malnutrition and survival. A total of 113 NPC patients were enrolled for nutritional assessment using the GLIM criteria at different radiotherapeutic time points, and related toxicities were graded. Regarding the results, 19 patients (16.8%) were malnourished before radiotherapy and 103 patients (91.2%) were malnourished at the end of radiotherapy. Among the phenotypic GLIM criteria, low fat-free muscle index (FFMI) before radiotherapy was associated with mucositis and radiodermatitis (p < 0.05). Importantly, patients with malnutrition before radiotherapy had significantly poorer 2-year progression free survival (PFS) than the patients being well-nourished (62.1% vs. 88.9%, p = 0.015). From the multivariate Cox regression model, being-well nourished before radiotherapy was the protective factor for PFS (HR: 0.27; 95%CI: 0.089-0.85; p = 0.023) and male was the risk factor for PFS (HR: 7.25; 95%CI: 1.548-34.00; p = 0.012). In conclusion, malnutrition according to the GLIM criteria is common in NPC patients undergoing radiotherapy, and pre-RT malnutrition is correlated with survival.Supplemental data for this article is available online at https://doi.org/10.1080/01635581.2022.2044059.