Background and Objectives: Nutrition and inflammation play a crucial role in the development of cancer. The prognostic value of the prognostic nutritional index (PNI) has been confirmed in some types of human cancers. This study analyzed the prognostic significance of the preoperative PNI in patients with stage III gastric cancer after curative surgery. Methods and Study Design: In this retrospective study, we enrolled 274 patients who underwent curative operation for stage III gastric cancer. The correlation between the preoperative PNI and overall survival (OS) was analyzed using Kaplan–Meier curves and multivariate Cox regression analyses. Results: The patients with a high PNI had a significantly higher median OS than did those with a low PNI (46.8 months vs 24.1 months, p=0.01). In the subgroup analysis, the survival benefit of the PNI was limited to the patients with poorly differentiated gastric cancer (high PNI, 46.8 months; low PNI, 21.8 months, p=0.004) and was not observed in those with well and moderately differentiated cancer (high PNI, 30.3 months; low PNI, 26.7 months, p=0.30). In the multivariate analysis, the PNI was an independent prognostic factor for OS. Conclusions: The PNI can be used as an independent prognostic biomarker for operable advanced gastric cancer.