BACKGROUND: The prognosis nutritional index (PNI) and the systemic inflammatory immunological index (SII) are characteristic indicators of the nutritional state and the systemic inflammatory response, respectively. However, there is an unknown combined effect of these indicators in the clinic. Therefore, the practicality of using the SII-PNI score to predict prognosis and tumor response of locally advanced gastric cancer (LAGC) following chemotherapy was the main focus of this investigation. METHODS: We retrospectively analyzed 181 patients with LAGC who underwent curative resection after neoadjuvant chemotherapy in a prospective study (NCT01516944). We divided these patients into tumour regression grade(TRG) 3 and non-TRG3 groups based on tumor response (AJCC/CAP guidelines). The SII and PNI were assessed and confirmed the cut-off values before treatment. The SII-PNI values varied from 0 to 2, with 2 being the high SII (≥ 471.5) as well as low PNI (≤ 48.6), a high SII or low PNI is represented by a 1 and neither is represented by a 0, respectively. RESULTS: 51 and 130 samples had TRG3 and non-TRG3 tumor responses respectively. Patients with TRG3 had substantially higher SII-PNI scores than those without TRG3 (p < 0.0001). Patients with greater SII-PNI scores had a poorer prognosis (p < 0.0001). The SII-PNI score was found to be an independent predictor of both overall survival (HR = 4.982, 95%CI: 1.890-10.234, p = 0.001) and disease-free survival (HR = 4.763, 95%CI: 1.994-13.903, p = 0.001) in a multivariate analysis. CONCLUSION: The clinical potential and accuracy of low-cost stratification based on SII-PNI score in forecasting tumor response and prognosis in LAGC is satisfactory.