Introduction: Oral malignant melanoma (OMM) is a rare malignant lesion of the oral mucosa. It accounts for 0.5% of oral cavity cancers and less than 1% of all melanomas. Most cases arise on the palate or gingiva. OMM is caused by unknown factors. Benign pigmentation may precede the neoplasm by several years. The malignant transformation of benign melanosis is poorly understood. Observation: The aim of this work is to present a new clinical case of oral malignant melanoma which appeared on benign melanosis with a brief review of the literature. A 37-year-old woman presented with a blackish pigmented plaque that covered the hard palate and vestibular maxillary gingiva and a soft, friable 2 cm nodule with ulcerated surface next to the 11, 12 and 13. Fifteen years ago, the patient underwent a biopsy that was in favor of benign melanosis. Unfortunately, the patient was followed for one year and then was lost. Recently, in front of the rapidity of the extension of the lesion, she came again. After biopsy, a final diagnosis of OMM is retained. Commentaries: OMM is often asymptomatic. It presents usually as a 1.5–4 cm, blackish grey, irregular, flat or nodular lesion. The neoplasm can appear on apparently normal oral mucosa and may be preceded by benign pigmented lesions. Few articles discussed malignant transformation of benign melanosis. OMM is characterized by its poor prognosis. The treatment of choice for OMM remains surgery with wide clear margins. Conclusion: Close monitoring is needed to detect signs of transformation and to early diagnose melanoma.