Struma ovarii is a rare form of ovarian tumor, represented in more than 50% by thyroid tissue. Malignant transformation is a rare presentation and comprises about 5% of cases. The authors report a female with papillary thyroid carcinoma arising in struma ovarii, diagnosed after surgery for a right ovarian mass. Thyroid examination showed two nodular lesions in the left lobe. Total thyroidectomy was performed and histology showed a papillary carcinoma, discordant in terms of ovarian tissue, synchronous to the ovary. Radioiodine ablation of residual thyroid tissue was performed and levothyroxine mildly-suppressive treatment was begun. Although the rareness of association, it is of extreme importance that patients with malignant struma ovarii undergo imaging of thyroid gland for study of coexistent disease.