Infectious mononucleosis is a self-limiting clinical syndrome caused by primary Epstein-Barr virus (EBV) infection. EBV-associated gastritis, however, has rarely been documented. We report a case of a 17-year-old woman who presented with fever, sore throat, and epigastric pain. Upper endoscopy revealed diffuse granular mucosae and elevated lesions in the stomach. Histologically, the biopsied mucosa was infiltrated by numerous atypical lymphocytes. From clinical, histopathologic, immunohistochemical, and in situ hybridization analyses, we diagnosed EBV-associated gastritis. Her symptoms spontaneously resolved, and follow-up endoscopy revealed improvement and no atypical lymphocytes. To prevent misdiagnosis and unnecessary treatment, the possibility of EBV-associated gastritis should be considered.