Preoperative prognostic nutritional index (PNI) has been widely demonstrated to predict survival of patients with malignant tumors. Its utility in predicting outcomes in patients with high-grade gliomas (HGG) remains undefined. A retrospective study of 188 HGG patients was conducted. An optimal PNI cut-off value was applied to stratify patients into high PNI (≥52.55, n = 78) and low PNI (<52.55, n = 110) groups. Univariate and multivariate analysis was performed to identify prognostic factors associated with overall survival (OS) and progression free survival (PFS). The resulting prognostic models were externally validated using a demographic-matched cohort of 130 HGG patients. In the training set, PNI value was negatively correlated with age (p = 0.027) and tumor grade (p = 0.048). Both PFS (8.27 vs. 20.77 months, p < 0.001) and OS (13.57 vs. 33.23 months, p < 0.001) were significantly worse in the low PNI group. Strikingly, patients in high PNI group had a 52% decrease in the risk of tumor progression and 55% decrease of death relative to low PNI. Multivariate analysis further demonstrated PNI as an independent predictor for PFS (HR = 0.62, 95% CI 0.43-0.87) and OS (HR = 0.56, 95% CI 0.38-0.80). The PNI retained independent prognostic value in the validation set for both PFS (p = 0.013) and OS (p = 0.003). On subgroup analysis by tumor grade and treatment modalities, both PFS and OS were better for the patients with high PNI. The PNI is a potentially valuable preoperative marker for the survival of patients following HGG resection.