A highly stable layered zirconium phosphate, (NH4)2[ZrF2(HPO4)2] (ZrP-1), was synthesized by an ionothermal method and contains an extremely dense two-dimensional hydrogen-bond network that is thermally stable up to 573 K, leading to combined ultrahigh water-assisted proton conductivities of 1.45 × 10-2 S cm-1 at 363 K/95% relative humidity and sustainable anhydrous proton conductivity of 1.1 × 10-5 S cm-1 at 503 K.