A 61-year-old woman was referred to the dental clinic because of a rapidly growing dark area along her gums. The discoloration had been present for several years but had expanded substantially in the past year. She had no bleeding from the area and no systemic symptoms. On physical examination, there were no skin findings. The intraoral examination revealed a large, flat, painless lesion with black and grayish-brown pigment along the right mandibular vestibular gingiva. An incisional biopsy revealed melanoma. The patient was referred to the oncology clinic for further evaluation. Computed tomography of the brain and maxillofacial region and whole-body positron-emission tomography revealed no lymph-node involvement or distant metastases. The patient underwent surgical resection of the lesion with negative margins. Melanomas may arise from the mucosal epithelial lining of the oral cavity and gastrointestinal tract. Oral melanoma is rare, and early diagnosis is critical, given that the condition may be asymptomatic until substantial progression has occurred. Four months after surgery, the patient remained under active surveillance with no signs of recurrence.