Graphene oxide (GO) contains abundant oxygen-containing functional groups acting as hydrogen bond acceptors for proton conduction on its basal plane. However, the dilemma in realizing bulk in-plane conduction and the metastability at room temperature of GO films both obstruct its application. Polyoxometalate-modified sponge-like GO monolith (PEGO) with 3D cross-linking inner structure, which exhibits unique shrink-expand effect to polar solvent, are synthesized. Owing to the introduction of polyoxometalates and the replacement of unstable epoxy groups by ethylenediamine, PEGO exhibits hitherto the highest proton conductivity under low relative humidity (1.02 x 10(-2) S cm(-1) at 60% relative humidity) and excellent long-term stability (more than 1 month). The outstanding conductivity originates from 3D transporting pathways, high-density hopping sites, and eliminated grain boundary resistance. This study provides a practical way to design GO-based proton-conducting material dominated by in-plane diffusion.