Water is composed of the electrochemically active species, H⁺ and OH^–, but has not been used as an active electronic material. In this study, a field-effect transistor is developed that uses water-infiltrated nanoporous glass as the gate insulator; this new application of water may be useful in electronics and energy storage. We fabricated a field-effect transistor structure on an oxide semiconductor, SrTiO₃, using water-infiltrated nanoporous glass amorphous 12CaO-7Al₂O₃ as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO₃ surface at room temperature. This leads to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration (10¹⁵-10¹⁶/cm²), which exhibits exotic thermoelectric behaviour.