BACKGROUND: Circulating exosomal microRNAs are an easily obtained and minimally invasive biomarker for cancer treatment. Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive carcinomas. It would thus be extremely crucial to predict therapeutic sensitivity and the patient prognosis in advance. METHODS: A search for miRNAs with a therapeutic biomarker in ESCC was performed using the miRNA expression signatures obtained from ESCC plasma exosomes before chemoradiotherapy. miR-191-5p was selected based on a comparison of miRNA signatures and the findings of previous reports. We explored the utility of circulating exosomal miR-191-5p as a prognostic biomarker of chemoradiotherapy along with its target gene, molecular pathway and functions specifically related to radiotherapy in ESCC. RESULTS: Overexpression of miR-191-5p promoted ESCC cell proliferation, invasion and migration. miRNA-191-5p overexpression promoted cell survival and reduced cell apoptosis after irradiation. Mechanistically, miR-191-5p may downregulate death-associated protein kinase 1 (DAPK1) to induce radiation resistance via the MAPK-JNK pathway. The 5-year progression-free survival rate for ESCC patients who underwent treatment, including radiotherapy with high circulating exosomal miR-191-5p expression was significantly lower than in those with a low expression. CONCLUSION: Tumor-derived exosomal miR-191-5p is a potential non-invasive biomarker for predicting the prognosis in esophageal cancer patients after radiotherapy.