In contrast to conventional ceramic ionic conductors relying on bulk ionic transport, making use of interfaces such as grain boundary and surface may provide various new possibilities to develop novel ionic conductors. Here we demonstrate that nanograined structures of yttriadoped zirconia (YSZ), of which the bulk property involves negligible proton solubility or conductivity, are endowed with appreciable proton conductivity via interfacial hydrated layers. A combination of nanopowder synthesis and ultra high-pressure compaction (4 GPa) at room temperature enables us to fabricate the nanograined specimens. The material thus prepared can retain an appreciable amount of protons and water within the grainboundary or "internal surface", resulting in a hierarchical structure of hydroxyl groups and water molecules with different thermal stability and thereby mobility. The physicochemical properties of those protonic species have been investigated by means of in situ FT-IR, 1H MAS NMR, and thermal desorption spectroscopy. At lower temperatures, proton conductivity prevails over normally observed oxide ion conductivity, which is facilitated by interplay of those protonic species at the interfaces. The present study provides a new prospect for developing proton-conducting materials which are based on "surface protonics" of nanograined oxides.