PURPOSE To evaluate the efficacy and safety of the radiosensitizer-sodium glycididazole (CMNa) combined with definitive concurrent chemoradiotherapy (CCRT) versus CCRT alone in patients with locally advanced esophageal squamous cell carcinoma (ESCC). METHODS Patients were enrolled and randomly assigned to receive CMNa plus CCRT (CMNa group) or CCRT alone (control group). CMNa (800 mg/m(2)) was administered three times per week during radiotherapy (1.8-2.0 Gy per fraction, total dose of 59.4-60.0 Gy). The primary endpoint was the objective response rate (ORR) and the secondary endpoint was the incidence of adverse events. RESULTS Sixty-six patients with ESCC were recruited from six medical centers between 2016 and 2018. The ORRs in the CMNa and control groups were 90.32% and 68.57%, respectively (chi(2) = 4.925, p = 0.0265). The proportions of patients with a complete response were 8/31 (25.81%) and 8/35 (22.86%) in the CMNa and control groups, respectively. The combination of CMNa and CCRT did not increase the incidence of acute or late adverse effects. CONCLUSION CMNa combined with CCRT improved the ORR in patients with ESCC compared with CCRT alone without increasing treatment-related toxicity. Trial Registration The protocol was registered at Clinicaltrials.gov (www.clinicaltrials.gov) (Clinical Trials ID: NCT02721563). Study Registration Date: March 29, 2016.