Mural nodules are rarely identified in cystic ovarian neoplasms, and have been categorized into sarcoma-like, sarcomatous, and anaplastic carcinomatous types. Most reports of these mural nodules have been described in mucinous ovarian tumors. In this case report, we describe an ovarian serous borderline tumor with mural nodules composed of high-grade carcinoma with anaplastic features and necrosis, including the morphologic features, immunoprofile, and results of tumor DNA sequencing. Omental involvement was also identified. Recognition of this phenomenon in serous tumors is important, so that thickened areas of cyst wall in ovarian serous tumors will be thoroughly examined.