Malignant neoplasms of the paranasal sinuses and nasal cavity are rare, comprising only 3% of all head and neck malignancies. This includes both primary sinonasal neoplasms and metastatic disease. We present the case of a patient with a maxillary soft tissue swelling, which proved to be a rare malignant tumor of maxillary sinus origin, a sinonasal undifferentiated carcinoma.