Autoimmune encephalitis (AE) is a condition of severe brain inflammation with a complex differential diagnosis. The identification of a specific neuronal antibody (NA) is not mandatory to diagnose AE. Moreover, even when a NA is detected, the clinical picture can be inconsequential (i.e., GAD-65) and not disease-specific (i.e., LGI1). Peculiar clinical manifestations and specific alterations of conventional tests as cerebral spinal fluid (CSF) and magnetic resonance imaging (MRI) can be sufficient to confirm the diagnostic suspicion of AE. New-onset seizures may be the first manifestation of AE and require immediate treatment. We report the case of a 19-year-old woman with sudden onset of focal motor seizures with unimpaired awareness, resistant to different intravenous antiseizure medications (ASMs). Ancillary tests (MRI, CSF analysis and electroencephalogram) were pathological and compatible with an autoimmune disorder of the brain. A weak positivity of GluR-3 antibody was detected in low serum dilution along with very high levels of angiotensin-converting enzyme in serum. After administration of high-dose corticosteroids, electro-clinical and neuroradiological pictures progressively normalized. This case report suggests that, even without a definite NA positivity, an inflammatory brain disorder of suspected autoimmune etiology should be considered based on clinical assessment and suggestive ancillary tests.