Optic neuritis is an inflammatory demyelinating disorder that primarily affects the optic nerve and is often associated with multiple sclerosis. While it is rare for optic neuritis to be accompanied by autoimmune encephalitis, it can occur in some cases. A 65-year-old woman with bipolar disorder presented with a progressively altered mentality. Magnetic resonance imaging of the brain showed no definite abnormal findings. Electroencephalography revealed nonconvulsive status epilepticus. Cerebrospinal fluid study and autoimmune and paraneoplastic encephalitis antibodies were negative. The patient was diagnosed with seronegative autoimmune encephalitis and treated with methylprednisolone, intravenous immunoglobulin, and rituximab. Her condition gradually improved except for persistent blindness on the left side. This case highlights the importance of considering autoimmune encephalitis even in the absence of identifiable pathogenic antibodies when clinical manifestations and response to immunotherapy support such a diagnosis.