Herein reported is a case of congenital malignant melanoma in a premature male baby from a 25-year-old healthy mother who was found to have hydramnios at the 29th week of gestation. The pregnancy was interrupted because of a large posterior neck mass detected by ultrasonography. The large neck mass of the baby was a malignant melanoma involving deep dermis and subcutaneous tissue. The skin over the mass showed a large area of pigmentation with hairs and the pigmentation involved the occipital scalp and posterior neck. Microscopically, the tumor cells were monotonous and showed polygonal and epithelioid appearance with prominent nucleoli indicative of malignant melanoma of a minimal deviation variety. Neither junctional components nor benign dermal nevus cells were noted. There were no distant metastasis or underlying leptomeningeal melanosis. This tumor is presumed to have developed from either preexisted congenital giant pigmented nevus with loss of benign components or de novo origin.