Mature cystic teratoma (MCT) of the ovary is a common benign tumor in women of reproductive age, with malignant transformation occurring in 1–2% of cases and transformation to apocrine carcinoma being extremely rare, with only four cases reported to date. The article describes a 68-year-old woman with a huge right ovarian tumor that had been growing slowly for more than 40 years. Imaging revealed a 35 cm cystic mass and the serum marker CA-125 was elevated at 559 U/mL. The patient underwent a laparotomy with the removal of 11 kg of tumor, histology confirmed MCT with malignant transformation into a triple-negative high-grade apocrine carcinoma, multifocally affecting the cyst wall without rupture of the capsule. Immunohistochemistry showed positive reactivity for AR and EGFR, negative for ER and PR, HER2 score 2+ (FISH negative); PET-CT indicated two reactive pericaval lymph nodes. The patient refused complex staging with hysterectomy, omentectomy and lymphadenectomy, the postoperative course was uneventful and at 6-month follow-up CA-125 was normal (<8 U/ml) without recurrence. Complete surgical excision is the mainstay of treatment because there is no standard systemic therapy; more cases are needed to better understand the tumor.