An 82-year-old patient presented a progressively growing hard thyroid nodule, and left ptosis. Additionally, ophthalmologic evaluation revealed ipsilateral miosis, diagnostic findings of Horner syndrome. Computerized tomography revealed a 7.5-cm thyroid mass infiltrating the main neck vessels. Although clinical and imaging data were suggestive of poorly differentiated thyroid carcinoma, fine-needle aspiration led to the diagnosis of papillary carcinoma. Paliative care was proposed to the patient due to the advanced stage of the neoplasm and to significant comorbidities. Horner syndrome is an infrequent manifestation of thyroid disorders and benign etiologies are more often implied. Malignant thyroid neoplasms represent a rare cause of Horner syndrome. However, an appropriate and prompt diagnosis is paramount for timely treatment of rare thyroid malignancies.