Monodisperse Ni nanoparticles (NPs) have been synthesized by the reduction of nickel(Ⅱ) acetylacetonate with the borane–tributylamine complex in a mixture of oleylamine and oleic acid. These Ni NPs are an active catalyst for the hydrolysis of the ammonia–borane (AB, H3NBH3) complex under ambient conditions and their activities are dependent on the chemical nature of the oxide support that they were deposited on. Among various oxides (SiO2, Al2O3, and CeO2) tested, SiO2 was found to enhance Ni NP catalytic activity due to the etching of the 3.2 nm Ni NPs giving Ni(Ⅱ) ions and the subsequent reduction of Ni(Ⅱ) that led to the formation of 1.6 nm Ni NPs on the SiO2 surface. The kinetics of the hydrolysis of AB catalyzed by Ni/SiO2 was shown to be dependent on catalyst and substrate concentration as well as temperature. The Ni/SiO2 catalyst has a turnover frequency (TOF) of 13.2 mol H2(mol Ni)–1min–1—the best ever reported for the hydrolysis of AB using a nickel catalyst, an activation energy of 34 kJ/mol ± 2 kJ/mol and a total turnover number of 15,400 in the hydrolysis of AB. It is a promising candidate to replace noble metals for catalyzing AB hydrolysis and for hydrogen generation under ambient conditions.