Malignant mixed mesodermal tumor (MMMT) of the ovary is a rare aggressive tumor that consists of an epithelial (carcinoma) and a stromal (sarcoma) component. MMMT accounts for less than 2% of ovarian cancers and has a very poor prognosis. We present a case and difficulties of diagnosing an ovarian MMMT in a postmenopausal woman with a history of invasive breast carcinoma treated postoperatively with radiotherapy and tamoxifen. A 52-year-old patient presented with unilateral ovarian tumor and moderately elevated CA125 (107 U/mL) and underwent laparotomy. Fine needle aspiration of the ovary and ascites for cytologic analysis, and tumor biopsy for histopathology were performed intraoperatively. Intraoperative cytologic sample showed necrotic background with rare single malignant cells with pale, abundant cytoplasm and conspicuous nucleoli suggesting clear cell carcinoma. Ascites sample showed inflammatory and reactive background with suspected papillary formations mimicking adenocarcinoma. Postoperatively, cytochemical PAS staining and immunocytologic staining with epithelial antigen (EA), cytokeratin (CK)7 and vimentin showed EA and PAS positivity for ovarian tumor, and EA and CK7 for ascites, suggesting a clear cell carcinoma. Histology revealed ovarian clear cell carcinoma. Three months later, the patient underwent hemicolectomy because of tumors on the right large bowel serosa with intraoperative morphological finding of metastatic malignant tumor without other specific features. Postoperative morphological analysis and immunohistochemical staining of the tumor revealed two malignant components, epithelial and stromal one. Repeat histologic analysis of the ovarian tumor confirmed ovarian MMMT (with a clear cell carcinoma component). Other studies of breast cancer emphasize that patients with invasive breast cancer and mutations of BRCA1 and BRCA2 genes are at an increased risk of primary ovarian cancer. Our study confirmed it and suggested considering a second primary malignant tumor of ovarian origin in patients with a history of breast carcinoma, postoperatively treated with radiotherapy and tamoxifen. Although rare, second primary ovarian tumors may present as MMMT.