An intracranial plasmacytoma is a rare form, which can involve the calvarium, dura or the cranial base. Only few case reports describe the manifestation of plasmacytoma of the skull base with affection of visual acuity. We describe the case of a 43-year-old woman, presenting with an acute unilateral loss of vision. The presumption diagnosis was retrobulbar neuritis as first manifestation of multiple sclerosis. MR imaging disclosed a tumour in the left orbital region and a meningioma was suspected. After complete resection with decompression of the optic nerve, the neuropathological examination revealed a lambda positive plasmacytoma. Additional work-up disclosed an involvement of multiple vertebral bodies. Due to the diagnosis of multiple myeloma, oncological therapy had been initiated. Skull base plasmacytoma is a rare disease. Solitary lesions causing neurological deficits should be treated aggressively including surgery for histological diagnosis and decompression of neural structures. Prognosis and further therapy depends on the systemic stage of disease, which has to be defined by diagnostic work-up.