Ammonia borane (AB) has received growing research interest as one of the most promising hydrogen-storage carrier materials. However, fast dehydrogenation of AB is still limited by sluggish catalytic kinetics over current catalysts. Herein, highly uniform and ultrafine bimetallic RhNi alloy nanoclusters encapsulated within nitrogen-functionalized hollow mesoporous carbons (defined as RhNi@NHMCs) are developed as highly active, durable, and selective nanocatalysts for fast hydrolysis of AB under mild conditions. Remarkable activity with a high turnover frequency (TOF) of 1294 molH2 molRh-1 min-1 and low activation energy (Ea) of 18.6 kJ mol-1 is observed at room temperature, surpassing the previous Rh-based catalysts. The detailed mechanism studies reveal that when catalyzed by RhNi@NHMCs, a covalently stable O-H bond by H2O first cleaves in electropositive H* and further attacks B-H bond of AB to stoichiometrically produce 3 equiv of H2, whose catalytic kinetics is restricted by the oxidation cleavage of the O-H bond. Compositional and structural features of RhNi@NHMCs result in synergic electronic, functional, and support add-in advantages, kinetically accelerating the cleavage of the attacked H2O (O-H bond) and remarkably promoting the catalytic hydrolysis of AB accordingly. This present work represents a new and effective strategy for exploring high-performance supported metal-based alloy nanoclusters for (electro)catalysis.