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Retroperitoneal teratomas constitute 5%–15% of all teratomas, with malignant transformation occurring in 3%–6% of cases, most commonly to squamous cell carcinoma. Adenocarcinoma of colorectal phenotype is exceedingly rare, with roughly 15 reported cases, one-third of which showed metastasis. Ovarian metastasis has not been previously documented; we report the first known case. A multiparous woman in her late 30s was diagnosed with a mature retroperitoneal teratoma that underwent malignant transformation into colonic-type adenocarcinoma with para-aortic lymph node metastases. She underwent surgical resection, including left nephrectomy and lymph node dissection, followed by adjuvant capecitabine plus oxaliplatin (CAPEOX) chemotherapy. Six months later, she developed ovarian metastasis, requiring additional surgery. She remains disease-free at the 1-year follow-up. This case illustrates the aggressive behaviour of retroperitoneal teratomas with adenocarcinoma transformation. Owing to its rarity, treatment is extrapolated from mucinous ovarian and colorectal cancer protocols. Optimal management includes surgery and systemic therapy, with long-term surveillance critical for detecting recurrence or delayed metastasis.