Nasosinusal carcinoma is an uncommon, highly aggressive tumor, occurring in the upper portion of the nasal cavity; has few symptoms and is difficult to diagnose; and is usually diagnosed because a larger tumor presents with a rapid course. The differential diagnosis of undifferentiated nasal carcinoma is broad and includes squamous cell carcinoma, nasopharyngeal carcinoma, neuroendocrine carcinoma, olfactory neuroblastoma, melanoma, and lymphoma. Because of the large number of differential diagnoses, further characterization is necessary, including microscopic examination and immunocytochemistry in order to make a definite diagnosis. A 20-year-old man had progressive bilateral nasal obstruction, anosmia, and frequent epistaxis for about 1 year. A computed tomography scan of the sinuses revealed a large lesion with ill-defined borders, suffering uptake of contrast medium, occupying the right nasopharynx, and extending into the right nasal cavity, as well as signs of light obstruction. Because it is a rare tumor without a defined protocol for best treatment, treatment includes surgical resection and adjuvant therapy, such as radiotherapy and chemotherapy, in order to improve the patient's survival and quality of life. The patient underwent endoscopic nasal surgery to resect the lesion that is currently performed in clinical ENT and oncology practices.