We reported a case of malignant melanoma and acquired dermal melanocytosis that appeared on congenital nevus spilus; this is the first report from Japan. An 85-year-old woman had had a nevus spilus on the right lower leg since birth. A black-brown nodule developed on the nevus three years before treatment. Blue-gray patches were found within the nevus on inspection. Histopathological analysis of these lesions revealed superficial spreading melanoma and acquired dermal melanocytosis, respectively. There have been 19 previous case reports of malignant melanoma on nevus spilus, and there have only been 4 cases of dermal melanocytosis (plaque-type blue nevus) on nevus spilus. We reviewed the reported cases in the literature and discussed the risk factors of nevus spilus.