High performance flexible hydrogen sensor using a silicon nanomembrane (Si NM) coated with palladium nanoparticles (Pd NPs) is developed. After the formation of gate structure on a released Si NM, selectively pre-doped Si NM is flip-transferred onto a plastic substrate. Along with Pd NPs deposited on top of the Si channel, the bottom gate structure allows the sensor to operate in a sub-threshold regime maximizing the response and recovery speed. A device simulation study revealed that the current change caused by shifting the threshold voltage upon H-2 exposure is the main operating mechanism of the sensor. The fabricated sensor shows high response (up to 250% @ 0.7% H-2 concentration), short response time (tau(10-90) = 10 s), and short recovery time (tau(90-10) = 10 s). In addition, the sensor shows low detection limit (50 ppm) and high mechanical robustness.