Simultaneous manipulation of topological and chemical structures to induce ionic nanochannel formation within solid electrolytes is a crucial but challenging task for the rational design of high‐performance electrochemical devices including proton exchange membrane fuel cell. Herein, a novel generic approach is presented for the construction of tunable ion‐conducting nanochannels via direct assembly of graphene oxide (GO)/poly(phosphonic acid) core–shell nanosheets prepared by surface‐initiated precipitation polymerization. Using this simple and rapid approach to engineer GO/polymer nanosheets at the molecular‐level, ordered and continuous nanochannels with interconnected hydrogen‐bonded networks having a favorable water environment can be created. The resulting membranes exhibit proton conductivities up to 32 mS cm−1 at 51% relative humidity, surpassing state‐of‐the‐art Nafion membrane and all previously reported GO‐based materials. A novel approach to construct tunable nanochannels via direct assembly of graphene oxide/polymer core–shell nanosheets is developed. Through molecular‐level engineering of the nanosheets, ordered and continuous nanochannels with well‐tailored chemical structures can be created. The resulting membrane exhibits a proton conductivity of 32 mS cm−1 at 51% RH, surpassing state‐of‐the‐art Nafion membrane and all previously reported GO‐based materials.