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Abstract Background Schlafen 11 (SLFN11) was recently identified as a dominant determinant of sensitivity to DNA-targeting agents including platinum-based drugs. SLFN11 also reportedly enhances cellular radiosensitivity. In this study, we examined the prognostic value of SLFN11 expression in esophageal squamous cell carcinoma (ESCC) patients treated with definitive chemoradiotherapy (dCRT), including the platinum derivative nedaplatin. Methods Seventy-three patients with ESCC who received dCRT were examined. SLFN11 expression was analyzed in pre-dCRT biopsies using immunohistochemistry and evaluated using a histo-score (H-score). Correlation between the H-score and overall survival was analyzed. An H-score ≥ 51 was provisionally defined as indicating high SLFN11 expression. Viability assays were performed using previously established isogenic human cell lines differentially expressing SLFN11 to test the usefulness of SLFN11 as marker of response to the dCRT regimen. Results High SLFN11 expression was independently associated with better prognosis in ESCC patients (hazard ratio = 0.295, 95% CI = 0.143–0.605, p = 0.001 for multivariate analysis). Kaplan-Meier survival curves showed that the prognostic value of high SLFN11 expression was most evident in patients at clinical stages II and III (p = 0.004). In in vitro study, SLFN11-proficient cells were highly sensitive to platinum derivatives compared to SLFN11-deficient cells. Conclusion SLFN11 expression is an independent prognostic factor for ESCC patients treated with dCRT and a potential biomarker for treatment selection of ESCC. Examination of SLFN11 may be particularly useful for clinical Stage II–III patients who wish to choose dCRT (instead of surgery) to preserve esophageal function.