Late transition metal nanoparticles (NPs) with a favorably high surface area to volume ratio have garnered much interest for catalytic applications. Yet, these NPs are prone to aggregation in solution, which has been mitigated through attachment of surface ligands, additives or supports; unfortunately, protective ligands can severely reduce the effective surface area on the NPs available for catalyzing chemical transformations. The preparation of 'metastable' NPs can readily address these challenges. We report herein the first synthesis of monodisperse metastable ruthenium nanoparticles (RuNPs), having sub 5 nm size and an fcc structure, in aqueous media at room temperature, which can be stored for a period of at least 8 months. The RuNPs can subsequently be used for the catalytic, quantitative hydrolysis of ammonia-borane (AB) yielding hydrogen gas with 21.8 turnovers per min at 25 °C. The high surface area available for hydrolysis of AB on the metastable RuNPs translated to an E_a of 27.5 kJ mol^-1, which is notably lower than previously reported values for RuNP based catalysts. Metal nanoparticles are becoming increasingly important in a variety of practical applications on account of their unique properties, especially for catalysis. Herein, we report the straightforward preparation of metastable ligand-free ruthenium nanoparticles (RuNPs) in aqueous solution that exhibit an fcc structure and which were found to be particularly active in the hydrolysis of ammonia borane (AB) at room temperature.