Cortisol is a pivotal hormone regulating stress responses and is linked to various health conditions, making precise and continuous monitoring essential. Despite their non-invasive nature, conventional cortisol detection methods often suffer from inadequate sensitivity and reliability at low concentrations, limiting their diagnostic utility. To address these limitations, this study introduces a novel paradigm for high sensitivity cortisol detection using field-effect transistor (FET) sensors with dual-gate (DG) structures. The proposed sensor platform enhances sensitivity through capacitive coupling without requiring external circuits. Cortisol detection performance was evaluated by immobilizing monoclonal antibodies activated via 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide onto a SnO₂ thin film-based extended-gate. The results revealed a sensitivity of 14.3 mV/dec in single-gate mode, which significantly increased to 243.8 mV/dec in DG mode, achieving a detection limit of 276 pM. Additionally, the reliability and stability of the sensor were validated by evaluating drift effects, confirming its ability to provide accurate detection even in artificial saliva environments containing interfering substances. In conclusion, the DG-FET-based cortisol detection approach developed in this study significantly outperforms conventional FET-based methods, enabling precise monitoring at ultra-low concentrations. This approach holds significant potential for diverse bioassays requiring high sensitivity and reliability in complex environments.