We report a case of a 12-year-old premenarcheal girl who developed ovarian mucinous adenocarcinoma. The tumor, which originated from the right ovary, weighed 1400 g, and had a smooth surface and thin capsule, was extirpated. The cut surface showed multilocular cysts of various sizes and yellowish gelatinous to solid parts that accounted for approximately 40% of the cut surface. The histological findings showed a continuous mixture of mucinous adenoma, mucinous borderline tumor, and mucinous adenocarcinoma with an expansile invasive pattern. It was assumed that an ovarian mass developed in an early period after birth or infancy, and the mutation of certain oncogens occurred in an early period, leading to cancer. While some etiological hypotheses concerning ovarian carcinogenesis have been suggested, we believe this case provides valuable information when considering a new onset mechanism of ovarian epithelial malignant tumors.