We present a case of adult-onset xanthogranuloma in the buccal mucosa. An 80-year-old woman was referred to our department for a mass in her buccal mucosa in July 2016. On clinical examination, a well-circumscribed, elastic-firm mass measuring 19 × 18 mm was detected on the right side buccal mucosa. A diagnosis of xanthogranuloma was established through biopsy, and the tumor was surgically removed in September 2016. Histopathological examination revealed sheets of histiocytes with foamy cytoplasm. Immunohistochemical examination demonstrated that the histiocytes were positive for CD68, but negative for S-100 protein. Thus, a diagnosis of xanthogranuloma was established based on the histopathology. The postoperative course has been uneventful for 2 years and 8 months, without tumor recurrence.