Phlegmonous infection involving the digestive tract has been reported to have a poor prognosis. However, the pathogenesis and clinical features of acute phlegmonous esophagitis have remained unclear due to the rarity of the disease. We herein report a case of acute phlegmonous esophagitis that showed a fulminant course during chemoradiotherapy for uterine cancer. The patient developed septic shock 10 h after postprandial nausea and vomiting, and a computed tomographic scan showed diffuse thickening of the esophageal wall. Severe leukopenia that was refractory to the administration of granulocyte colony-stimulating factor persisted during the first few days. The patient fortunately survived after intensive treatment. The acute phlegmonous esophagitis of the present case might have been evoked and worsened by chemoradiotherapy due to its emetic and myelosuppressive adverse effects, respectively. Although its incidence is extremely rare, acute phlegmonous esophagitis may occur as a life-threatening complication of chemoradiotherapy.