Ovarian teratomas are a common type of ovarian neoplasm. These tumors have various histologic subtypes, including struma ovarii (SO), which is considered a rare tumor. SO is characterized by the presence of thyroid tissue within its components. Since it contains thyroid cells, it can produce thyroid hormones, leading to thyrotoxicosis, or it can transform into malignant tissue, resulting in thyroid carcinoma, with papillary thyroid cancer being the most common histologic subtype. We present the case of a 46-year-old woman who experienced abdominal pain and was found to have an ovarian mass with malignant features. Excision of the ovaries revealed SO with well to moderately differentiated papillary thyroid cancer. The patient's thyroid function tests were normal, and no distant metastases were found. She was started on a small dose of supplemental thyroid hormone to suppress thyroid-stimulating hormone (TSH) to a low normal level. The highest incidence of malignant struma ovarii (MSO) occurs in the fifth decade of life. The symptoms are usually vague and nonspecific, and the diagnosis is typically made after excision of the adnexal mass. The management of SO is primarily surgical, which is curative in most cases, but should be preceded by tumor staging and postoperative thyroid hormone suppression, along with surveillance using imaging and thyroid biochemical markers.