Sellar plasmacytomas are rare tumors arising from plasma cells. They are often misdiagnosed as adenomas. We report the case of a 63-year-old woman with headache, cranial nerve III palsy and decreased visual acuity. Imaging revealed an extensive lesion centered on the clivus, extending to the cavernous sinus bilaterally and into the sphenoid sinus. The hormonal tests were compatible with panhypopituitarism and mild hyperprolactinemia. The first hypothesis was invasive pituitary adenoma. Partial resection was achieved, and the immunohistochemical evaluation was compatible with plasmacytoma. After a few weeks, she developed lumbar and hip pain, and the imaging confirming osteolytic lesions. The final diagnosis was multiple myeloma.