Malignant mixed Mullerian tumour commonly known as carcinosarcoma is a rare and aggressive biphasic tumour with epithelial and mesenchymal components. In this reported case, a 68 year old post menopausal, nulliparous female, presented with complaints of difficulty in breathing and abdominal discomfort. Clinical evaluation and radiological findings were consistent with malignancy of left ovarian origin. Patient was taken up for total abdominal hysterectomy with bilateral salpingo-oophorectomy. But intraoperatively, the tumour was found as a single large retroperitoneal mass with no connection to ovaries or other pelvic organs. Histopathological examination of the specimen revealed features of malignant mixed Mullerian tumour. Immunohistochemical markers showed positivity for CK 20, Pan CK and EMA in carcinomatous areas and vimentin in sarcomatous areas. ER and PR were found to be negative. Patient was on regular follow-up and responded well to chemo and radiotherapy. Primaries in retroperitoneum also have poor prognosis similar to carcinosarcomas of female genital tract.