ABSTRACT Among 9457 aquaporin‐4 (AQP4) immunoglobulin G‐positive patients, 2071 underwent autoimmune encephalitis antibody testing and 85 (4%) had coexisting antibodies, most commonly cerebrospinal fluid glial fibrillary acidic protein‐IgG and N‐methyl‐D‐aspartate receptor‐IgG, or serum high‐titer GAD65‐IgG. Compared with AQP4‐IgG positive neuromyelitis optica spectrum disorder controls, dual‐positive patients were younger and more frequently exhibited encephalopathy, psychiatric symptoms, seizures, ovarian teratomas, and deep gray matter, temporal lobe, or diencephalic MRI abnormalities. GAD65‐IgG‐positive patients developed stiff‐person syndrome. These findings define a distinct mixed glial‐neuronal autoimmune phenotype and support additional neural antibody testing in atypical AQP4‐IgG‐positive presentations.