The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics. Especially in recent years, much effort has been devoted to the fundamental understanding and practical applications of transistor configurations with EDLs because of their ability for high-density charge accumulation. However, to exploit additional new functionalities of such an emerging interface is not only of great importance but also a huge challenge. Here, we demonstrate that, by introducing protic ionic liquid (PIL) as the gate dielectric for ZnO EDL transistors (EDLTs), small and chemically active ions, such as protons and hydroxyls, can serve as an adsorption medium to extend the interfacial functionalities of EDLTs. By selectively driving the H+ or OH- groups onto ZnO channel surfaces with an electric field, the charged adsorbates interact with surface atoms in different adsorption mechanisms, showing remarkable variations in electron transport and providing a possibility for the recognition of surface polarity. Most significantly, the large hysteresis in the transfer characteristics of PIL-EDLTs makes the device available and promising for nonvolatile proton memory devices via surface hydrogenation and dehydrogenation processes. Such a finding provides us with new opportunities to understand liquid/solid heterogeneous interface phenomena and to extend the practical functions of EDLs through controllable interfacial interaction.