 In this study, participants with histologically diagnosed locally advanced esophageal squamous cell carcinoma who have received preoperative cisplatin-based chemoradiotherapy followed by surgery harbouring high risk of tumor recurrence will receive adjuvant cisplatin-based chemoradiotherapy followed by pembrolizumab. The primary study hypothesis is that adjuvant pembrolizumab will improve the 1-year recurrence-free survival rate compared to historical control. 