Oral melanotic lesions, including melanin pigmentation, melanocytic nevus, and malignant melanoma, are well-recognized pathologic entities. However, melanocytic proliferation within malignant oral mucosa] lesions is not well documented. We report the unusual case of a 53-year-old Japanese man who developed oral carcinoma in situ (CIS) with melanocytic proliferation and melanin pigmentation in the epithelial layer. The patient, a nonsmoker and an opportunistic drinker, presented with a brown tongue lesion. Initial examination found a large brown pigmented area and multiple small white patchy areas on the right tongue border. The pigmentation had an ill-defined border with uneven color distribution. Physical examination found no abnormalities. Ultrasonography did not find a deeply infiltrating lesion. Oral mucosal malignant melanoma in situ was diagnosed, and partial tongue resection was performed. Histopathologic examination found oral pigmented CIS. To the best of our knowledge, this is only the third reported case of oral pigmented CIS.