BACKGROUND AND OBJECTIVES: Seronegative autoimmune encephalitis (SNAIE) is an enigmatic entity which lacks detectable autoantibodies, leading to a diagnostic delay. Our objective was to study the frequency of SNAIE and compare the clinical, electrophysiological, and imaging characteristics of patients with SNAIE to those in patients with seropositive autoimmune encephalitis (SPAIE). METHODS: This ambispective observational study was undertaken at a tertiary care center in South India for 7 years. The demographic data, clinical features, cerebrospinal fluid (CSF) analysis findings, electroencephalogram (EEG), and 3 T magnetic resonance imaging (MRI), and 18F fluoro-2-deoxy-d-glucose positron emission tomography computerized tomography (18F FDG PET-CT) findings were compared. RESULTS: Of the 64 subjects enrolled, SNAIE frequency was 62.5%, with a notable 3:1 male-to-female ratio. Seizure was the predominant clinical presentation in SNAIE compared to SPAIE (50% vs. 20%, P value 0.02). Refractory seizures (45% vs. 4.5%) and status epilepticus (37.5% vs. 0%) were more common in SNAIE ( P value 0.002), whereas involuntary movements and faciobrachial dystonic seizures were common in SPAIE (66% vs. 16%, P value 0.04). CSF pleocytosis was frequently seen in SPAIE (50% vs. 17%, P value 0.006). EEG findings in SNAIE included increased epileptiform discharges and built-up rhythms. FDG PET abnormalities were observed in both groups (91% SPAIE, 97% SNAIE), but the seropositive group exhibited more temporal lobe involvement (58% vs. 30%, P = 0.036) and SNAIE had more diffuse involvement. CONCLUSIONS: SNAIE exhibited male predominance, primarily presenting with isolated seizure as the initial presentation. EEG and FDG PET findings distinguished the two groups, emphasizing the importance of these modalities as biomarkers in the early detection and management of SNAIE.