Background <p>Anaplastic thyroid carcinoma (ATC) is a rare, undifferentiated thyroid malignancy with a poor prognosis and rapid clinical progression. It accounts for less than 10% of thyroid cancers but contributes disproportionately to thyroid cancer-related mortality. Histologically, ATC exhibits significant pleomorphism and can present with diverse morphological patterns, often coexisting with differentiated thyroid carcinoma.</p> Case Presentation <p>We report a case of a 39-year-old female with a 20-year history of anterior neck swelling, which rapidly progressed over six months. Imaging revealed a heterogeneously enhancing lesion in the right thyroid lobe with extrathyroidal extension and cervical lymphadenopathy. The patient underwent total thyroidectomy with central and lateral neck dissection. Histopathological examination demonstrated a high-grade malignant tumor composed of spindle-shaped, pleomorphic, and multinucleated giant cells with focal osteoid formation, consistent with ATC. A coexistent focus of papillary thyroid carcinoma was also identified. Immunohistochemistry revealed PAX8 negativity and SATB2 positivity, supporting its emerging role as a diagnostic marker for ATC.</p> Conclusion <p>This case highlights an unusual presentation of ATC in a relatively young patient. The presence of osteoclast-like giant cells and osteoid formation adds to the histological complexity. Accurate diagnosis requires a multidisciplinary approach integrating clinical, radiological, histopathological and immunohistochemical findings. Early recognition and surgical intervention are crucial given the aggressive nature and rapid progression of ATC. Emerging biomarkers like SATB2 may enhance diagnostic sensitivity, especially in challenging morphological variants.</p>