Aim: To explore the clinical utility of the systemic immune-inflammation index (SII) for predicting the prognosis of esophageal squamous cell carcinoma (ESCC). Patients & methods: After calculating the SII in 180 patients with ESCC, the relationship between SII values and the pre-/post-radiotherapy SII ratio and overall survival was determined. Results: The median overall survival was 649 days for the entire group and 909 and 466 days for the high and low pre-/post-radiotherapy SII ratio groups, respectively. Multivariate analysis identified Karnofsky performance status (p = 0.045), lymphatic metastasis (p = 0.032), mid-radiotherapy SII (p < 0.001) and pre-/post-radiotherapy SII ratio (p = 0.003) as independent prognostic factors. Conclusion: The pre-/post-radiotherapy SII ratio and mid-radiotherapy SII are potentially effective markers for predicting ESCC prognosis. Lay abstract The systemic immune-inflammation index (SII) is calculated from the counts of peripheral blood platelets (P), neutrophils (N) and lymphocytes (L) per liter according to the formula SII = P x N/L. The SII is associated with poor survival in certain cancer types. However, some reports have examined the prognostic value of the SII in patients with ESCC who were undergoing radiotherapy or radical chemoradiotherapy. As such, the current study sought to investigate the clinical prognostic value of the SII during radiotherapy and the ratio of the SII before and after radiotherapy in patients with ESCC who were undergoing chemoradiotherapy or radiotherapy. The study found that the pre-/post-radiotherapy SII ratio and mid-radiotherapy SII are potentially effective markers for predicting ESCC prognosis.