: Glioblastoma (GBM) is the most common primary brain tumor in adults, with a poor prognosis and survival. Although typically presenting with focal neurological deficits, seizures, or cognitive decline, GBM can occasionally mimic autoimmune encephalitis (AE), leading to significant diagnostic delay. The overlap in clinical, radiological, and serological findings between GBM and AE underscores the need for thorough evaluation. : We retrospectively reviewed cases of patients diagnosed between 2016 and 2023 with pathology-confirmed GBM, critically rethinking those cases initially diagnosed with AE at symptom onset. The diagnostic workup included magnetic resonance imaging (MRI), cerebrospinal fluid (CSF) analysis, autoantibody testing, and whole-body nuclear scanning to exclude extracranial malignancies. : We found five female patients diagnosed with GBM who initially presented with signs and symptoms suggestive for AE. Initial MRI showed non-specific brain tissue alterations, without definitive tumor features. CSF analysis was largely unremarkable, though some cases exhibited positive autoantibodies. Despite therapy, clinical deterioration and follow-up MRI revealed infiltrative intra-axial lesions with contrast enhancement, leading to pathology-confirmed GBM diagnoses. All patients had poor prognoses, with a mean survival of 10 ± 4 months. : GBM can mimic AE, delaying appropriate treatment. In patients with atypical MRI findings and suboptimal response to therapy, early follow-up imaging and biopsy should be considered to exclude malignancy. A multidisciplinary approach is critical for timely diagnosis and improved management.