This report aims to share a very uncommon case of primary manifestation of multiple myeloma as a sinonasal tumor. CASE REPORT: This is a patient of 74 years old, previously asymptomatic, who sought the emergency room of our hospital with reports of loss of visual acuity on the right for three days. RNM performed by neurosurgery team, showing: mass located at the posterior ethmoid cells to the right posterior inferior invading the upper wall of the sphenoid sinus, medially reached the right orbital apex involving the optic nerve, subsequently reaching the anterosuperior right cavernous sinus. It was held sternotomy access right by nasal endoscopy with biopsy. Pathological examination revealed undifferentiated carcinoma, with indication of immunohistochemistry for diagnosis. Finally, it proved to treat Multiple Myeloma, and the patient was referred for evaluation and monitoring by the team of Oncology. CONCLUSION: Plasmocytoma is a rare neoplasm originating from B lymphocytes It may present as three variants: multiple myeloma, solitary bone plasmocytoma and extramedullary plasmocytoma, and pathology of various manifestations and requires early diagnosis and treatment for better prognosis.