Background and clinical significance: Autoimmune encephalitis (AE) is an inflammatory brain disorder that manifests through a diverse, unspecific range of neuropsychiatric symptoms. When AE occurs alongside a primary neurodegenerative disorder, the shared symptoms can create a mixed clinical profile, making diagnosis more difficult and potentially postponing effective management and treatment. Case presentation: We describe the case of a 58-year-old female with a one-year history of progressive behavioral and personality changes who presented a subacute confusional state, psychomotor retardation alternating with psychomotor agitation, apathy, visual hallucinations, and motor symptoms. Examination revealed Parkinsonian symptoms and frontal lobe signs. Neuroimaging showed frontotemporal atrophy, while cerebrospinal fluid analysis excluded infection but demonstrated elevated phosphorylated tau, supporting an underlying neurodegenerative process. An electroencephalogram revealed asymmetric temporal slowing without overt epileptiform activity. An initial diagnosis of behavioral variant frontotemporal dementia (bvFTD) was established. Due to rapid clinical deterioration and fluctuating cognition, autoimmune testing was expanded to a full antibody panel, which identified elevated serum anti-glutamic acid decarboxylase 65 (anti-GAD65) antibodies (60 UI/mL, reference range 0-5 UI/mL), establishing a possible coexisting diagnosis of anti-GAD65 autoimmune encephalitis. Initial treatment with intravenous immunoglobulin produced minimal improvement; however, therapeutic plasma exchange led to the remission of psychosis and significant improvement in rigidity, bradykinesia, and attention, with modest amelioration in global cognition. Conclusions: This case highlights the diagnostic challenges posed by overlapping AE and bvFTD clinical pictures, especially when neurodegenerative features obscure an underlying autoimmune process. Early, panel-based neural antibody testing-and consideration of AE even in patients already diagnosed with a major neurocognitive disorder-is critical for avoiding delays in immunotherapy. Prompt recognition and treatment of AE may substantially improve clinical outcomes, even in complex cases with suspected overlap.