AbstractBackgroundThere are more than 30 immunoglobulin G (IgG) autoantibody biomarkers of autoimmune cerebellar ataxia reported.ObjectiveTo describe a novel IgG biomarker targeting neurobeachin, a scaffolding protein integral to synapse formation and function.MethodsClinical and laboratory‐based assessment was used for a novel diagnostic biomarker of autoimmune ataxia.ResultsA 46‐year‐old woman had ataxia onset followed by bilateral knee pain, attributable to rheumatoid arthritis. She was negative for standard neural antibodies. Her ataxia stabilized and tremor initially improved after corticosteroid treatment, but she progressed to wheelchair‐dependence (on methotrexate and then rituximab) over 4 years, in keeping with the trajectory encountered by many with autoimmune ataxia. In contrast, her inflammatory arthritis remitted with immunotherapy. Neurobeachin‐IgG was unmasked in serum and cerebrospinal fluid using protein arrays and confirmed using neurobeachin‐specific confocal immunofluorescence and Western blot.ConclusionsNeurobeachin‐IgG is a new neural antibody biomarker of cerebellar ataxia among a growing antibody repertoire, collectively common but often individually rare. © 2024 International Parkinson and Movement Disorder Society.