With the advent of immune checkpoint inhibitors (ICI), cancer treatment options have widened in recent years. However, ICI-specific adverse events (irAEs) have been reported. Lower gastrointestinal lesions, such as colitis and enteritis, account for most gastrointestinal irAEs, and reports of upper gastrointestinal lesions are rare. We report a rare case of gastroesophagitis associated with ICI. The patient was a 64-year-old male. He was diagnosed with lung adenocarcinoma stage IIIB (cT2aN3M0), and pembrolizumab (PEM) was started as a first-line treatment. Severe gastroesophagitis with laryngopharyngitis was confirmed 5 months after PEM administration. These improved after withdrawal of PEM and steroid administration. Reports of ICI-associated gastritis remain limited, especially with laryngopharyngitis;therefore, we consider this case as valuable, in which we confirmed the clinical features of ICI-associated gastroesophagitis and its therapeutic effects.