Acute phlegmonous esophagitis is a rare life-threatening disease that often requires surgical intervention in case of complications, including esophageal abscess, perforation, or mediastinitis. We present a case of acute phlegmonous esophagitis, in which magnetic resonance imaging (MRI) proved useful in planning the treatment strategy. An 89-year-old woman was admitted to the emergency department with painful swallowing and respiratory distress. She was diagnosed with acute phlegmonous esophagitis and a hypopharyngeal abscess based on computed tomography (CT) findings. However, there was a discrepancy between the clinical course and CT findings. Given the improvement of the patient’s condition with conservative treatment with ampicillin/sulbactam, the CT findings suggested an apparent abscess due to increased esophageal wall thickness. However, MR diffusion-weighted images showed a slightly high-intensity signal, suggesting that the enlargement was due to edema rather than an abscess. The patient recovered successfully following conservative treatment. Thus, our findings demonstrate the utility of MRI in the treatment planning of acute phlegmonous esophagitis, especially in cases with unreliable contrast-enhanced CT findings. However, future studies are warranted to explore the utility of MRI in the management of such cases.