A 48 year-old man presented with typical symptoms and signs of optic neuritis right eye. However, vision deteriorated to no light perception with the development of retinal vascular occlusions despite treatment with intravenous steroids. Serial MRI demonstrated enlarging lesions involving right optic nerve/chiasm, corpus callosum, and cerebellum. A superficial optic nerve biopsy showed atrophy. Cerebellar biopsy revealed the diagnosis of anaplastic astrocytoma (WHO Grade III). The case demonstrates that malignant optic nerve glioma of adulthood may be multicentric in nature and mimic optic neuritis clinically. We recommend a full-thickness optic nerve biopsy to help avoid a false-negative outcome.