Autoimmune encephalitis (AE) associated with autoantibodies against γ-aminobutyric acid-B receptor (GABABR-AE) is frequently identified in middle-aged and elderly males. The disease is characterized by seizures, mental, and behavioral abnormalities, as well as recent memory decline. Anti-GABABR antibody-associated encephalitis, presenting with syncope as the first symptom is rare. Here we report a case of AE with syncope as the first symptom. A 55-year-old male presented to the emergency department with transient loss of consciousness, initially diagnosed as syncope. As the disease progressed, the patient exhibited seizures, abnormal mental behavior, and cognitive impairment. Ultimately, the patient was diagnosed with right lung small cell lung cancer. The initial atypical symptoms and the lack of clear imaging features of GABABR encephalitis hinder early diagnosis. This case highlights the importance of screening for the underlying etiology of syncope in middle-aged and elderly patients.