Ulceration due to cytomegalovirus (CMV) infection rarely occurs in the oral cavity. The aim of this report was to document a case of CMV oral ulcer. The Thai female patient was present with pain at the left retromolar area. The patient was HIV-positive. Oral examination revealed an ulcer at the left retromolar mucosa. Histopathologically, the specimen showed characteristic features of CMV infection; enlarged endothelial cells with intranuclear and intracytoplasmic inclusion bodies. Because of her economic status, the supportive treatment was provided to alleviate the pain.Ulceration due to cytomegalovirus (CMV) infection rarely occurs in the oral cavity. The aim of this report was to document a case of CMV oral ulcer. The Thai female patient was present with pain at the left retromolar area. The patient was HIV-positive. Oral examination revealed an ulcer at the left retromolar mucosa. Histopathologically, the specimen showed characteristic features of CMV infection; enlarged endothelial cells with intranuclear and intracytoplasmic inclusion bodies. Because of her economic status, the supportive treatment was provided to alleviate the pain.