Oral Malignant Melanoma (OMM) is a rare, aggressive neoplasm of melanocytic origin, which is known to have the worst prognosis than that of cutaneous melanomas. The five-year survival reported in the literature for OMM varies from 0 - 45 % whereas the overall survival for head and neck melanomas ranges between 20 and 48%. Maxillary gingiva and palate are commonly affected. Very few cases have been reported in the mandibular gingiva. It can occur at any age with the range of 20 to 80 years, but less common below 30 years. OMM may appear in various forms including pigmented macule, pigmented nodule, or a large pigmented exophytic lesion or an amelanotic variant of any of these three forms. Here we are reporting a rare case of large exophytic, multilobulated OMM involving whole of left mandibular gingiva in a 40 year old male patient.