The Global Leadership Initiative on Malnutrition (GLIM) is a new approach established for the assessment of malnutrition. This study aimed to validate the GLIM for the diagnosis of malnutrition in patients with type 2 diabetes mellitus (T2DM) in Saudi Arabia, using the Subjective Global Assessment (SGA) as a reference. In addition, the association between the GLIM criteria and vascular complications in those patients was examined. A cross-sectional study was conducted on 101 patients with T2DM. The level of agreement between the GLIM and SGA tools was calculated using the kappa coefficient (κ). A receiver operating characteristic curve was used to determine the sensitivity and specificity of the GLIM. In addition, binary logistic regression was performed to investigate the association between each GLIM criterion and T2DM vascular complications. According to both the GLIM and the SGA, malnutrition was found in 15.8% and 17.8% of patients, respectively. The GLIM criteria achieved a very good level of accuracy (AUC = 0.877). The agreement between the tools was substantial (κ = 0.778). The ‘disease/inflammation’ criterion of the GLIM was significantly associated with macrovascular complications. To conclude, the GLIM criteria for diagnosis of malnutrition presented satisfactory levels of validity, and as such are acceptable for assessing the nutritional status of patients with T2DM.