Carcinosarcomas are rare malignant neoplasms formed by embryonic tissues (ectoderm and mesoderm) and present remarkably aggressive character with unfavourable prognosis. These lesions are rarely diagnosed in the sinonasal cavity and only a few cases are reported in the literature. This manuscript aimed to report a rare case of a large and aggressive sinonasal carcinosarcoma that involved the facial middle third and to discuss the proposed treatment.A 54-year-old female patient sought treatment for a large swelling on the left side of the face. A CT-scan revealed an expansive hypodense image in the facial middle third. An incisional biopsy was performed under local anaesthesia and the material was sent for histopathological analysis.Due to the extremely aggressive character of carcinosarcomas, the optimal management of carcinosarcomas remains uncertain and a challenge for clinicians. In this case report, the prognosis of such a large tumour became unfavourable since the patient sought late for treatment. Therefore, suspicion and early detection are crucial for improving the prognosis and quality of life of the patients with such neoplasia.Besides being rare and extremely aggressive, sinonasal carcinosarcomas present worse patient survival when compared to other carcinosarcomas in the head and neck region. Thus, this report may contribute to a better understanding of this tumour behaviour.