Although radiation esophagitis is one of the most common adverse events that occurs during chemoradiotherapy (CRT) in patients with esophageal cancer, CRT-associated cytomegalovirus (CMV) esophagitis is rare. CMV esophagitis typically occurs in patients with an immunosuppressed status. Here we report a case of CMV esophagitis during CRT initially treated as radiation esophagitis. A 64-year-old man with mid-thoracic esophageal cancer was admitted to our hospital with clinical stage cT4bN1M1 (supraclavicular lymph node metastasis) Stage IV according to the UICC ver. 7 guidelines, and he was administered definitive concurrent CRT. From the 39th day of CRT onwards, he presented with a sustained fever and severe odynophagia that was resistant to antibiotic therapy. An esophagoscopy revealed severe esophagitis with a circumferential ulcer throughout the entire esophagus, and CMV esophagitis was clinically suspected because of positive result of CMV antigenemia. Subsequently, antiviral therapy for CMV provided dramatic relief of his symptoms. Later, CMV DNA was confirmed with a polymerase chain reaction in the biopsy specimen.The symptoms of CMV esophagitis resemble those of radiation esophagitis and can make the diagnosis difficult. Thus, CMV esophagitis associated CRT may be overlooked or masked by radiation esophagitis and can cause a delay in healing. Therefore, CMV esophagitis may be considered when severe intractable esophagitis is observed during CRT.