Malignant transformation of a mature cystic teratoma (MCT) of the ovary is rare, with squamous cell carcinoma (SCC) being the most common type. The authors report a novel case of microinvasive squamous cell carcinoma arising in a mature cystic teratoma of the ovary. A 56-year-old woman presented with a 12-cm mass, which was diagnosed as a left ovarian mature cystic teratoma preoperatively by ultrasonography. Subsequently, laparoscopic surgery for the ovarian tumor was performed. The pathologic diagnosis was microinvasive squamous cell carcinoma arising in a mature cystic teratoma of the ovary. Appropriate staging surgery was then performed, with no evidence of malignant tissue except for the removed left ovary. Microinvasive SCC arising in MCT of the ovary is extremely rare, and, to the best of the authors' knowledge, this has not previously been reported in the literature.