Ammonia-borane (NH₃BH₃, AB) is an valuable chemical hydrogen storage material due to its large hydrogen content of 19.6 wt%, superior stability, and environmental benignity. Highly active catalysts play an indispensable role in hydrogen production from AB hydrolysis. However, it remains a big challenge to achieve that kind of catalyst, i.e., small uniform size and homogeneous dispersion of metal particles on an oxide support. In this work, nano-cube ceria was successfully synthesized through a hydrothermal method. Two different catalysts, Ru/CeO₂–NaBH₄ and Ru/CeO₂–H₂/Ar, were prepared by loading ultrafine ruthenium nanoparticles on nano-cube ceria through the reduction of Ru³⁺ ions by NaBH₄ in solution and H₂/Ar mixed gas, respectively. The characterization results show that the ruthenium nanoparticles in a very narrow size range of 0.90–2.26 nm are uniformly dispersed on the surface of nano-cube ceria with the existence of Ce³⁺ and rich oxygen vacancies, thus providing a strong metal–support interaction. Therefore, even with a small amount of ruthenium (1.6 wt%), Ru/CeO₂–H₂/Ar exhibits a relatively low activation energy (E_a) of 42.98 kJ mol⁻¹ and high catalytic activity with a turnover frequency (TOF) of 176.9 mol_H2 mol_Ru⁻¹ min⁻¹ for the catalytic hydrolytic dehydrogenation of AB. Additionally, about 65.8% of catalytic activity is retained after five consecutive tests. This work provides a general strategy to achieve high-performance supported catalysts with ultrafine metal nanoparticles in a uniform distribution.