Hydrolysis of ammonia borane (AB) is a very convenient source of H2, but this reaction needs catalytic activation to become practical under ambient conditions. Here this reaction is catalyzed by bimetallic late transition-metal nanoparticles (NPs) that are stabilized and activated by “click” dendrimers. Dendrimers 1 and 2 contain 27 or 81 triethylene glycol terminal groups and 9 or 27 1,2,3-triazole ligands, respectively, located on the dendritic tethers. A remarkable synergy between Pt and Co in the Pt–Co/“click” dendrimer nanocatalysts is revealed. These Pt–Co/“click” dendrimer catalysts are much more efficient for hydrolysis of AB than either “click” dendrimer-stabilized Co or Pt analogues alone. The best catalyst, Pt1Co1/1, stabilized by the nonatriazole “click” dendrimer 1 achieves a turnover frequency number (TOF) of 303 molH2 molcat–1 min–1 (606 molH2 molPt–1 min–1) at 20 ± 1 °C. The AB hydrolysis reaction catalyzed by Pt1Co1/1 is boosted by NaOH, the TOF value reaching 476.2 molH2 molcat–1 min–1 (952.4 molH2 molPt–1 min–1), one of the very best results ever obtained for this reaction. The presence of ≥25% Pt in the CoPt nanoalloy provides a reaction rate higher than that obtained with the pure PtNP catalyst alone. The kinetics involves in particular a kinetic isotope effect kD/kH of 2.46 obtained for the hydrolysis reaction with D2O, suggesting that an O–H bond of water is cleaved in the rate-determining step. Tandem reactions were carried out for the hydrogenation of styrene with hydrogen generated from the hydrolysis of AB. Performing this tandem reaction with D2O shows deuteration of the ethylbenzene products, confirming O–D bond cleavage and H/D scrambling on the bimetallic NP surface. Finally, a full reaction mechanism is proposed. This dramatic synergy type should also prove to be useful in a number of other catalytic systems.