Despite curative-intent treatment, most patients with locally advanced esophageal cancer will experience disease recurrence or locoregional progression, highlighting the need for new therapies. Current guidelines recommend definitive chemoradiotherapy in patients ineligible for surgical resection, but survival outcomes are poor. Pembrolizumab is well tolerated and provides promising antitumor activity in patients with previously treated, advanced, unresectable esophageal/esophagogastric junction cancer. Combining pembrolizumab with chemoradiotherapy may further improve outcomes in the first-line setting. Here, we describe the design and rationale for the double-blind, Phase III, placebo-controlled, randomized KEYNOTE-975 trial investigating pembrolizumab in combination with definitive chemoradiotherapy as first-line treatment in patients with locally advanced, unresectable esophageal/gastroesophageal junction cancer. Overall survival and event-free survival are the dual primary end points. Clinical trial registration: NCT04210115 (ClinicalTrials.gov).