Stable Au and Agnanoparticles with diameter 5–10 nanometers were synthesized in water at pH 7 and at room temperature by a novel green photocatalytic method. The method features very low energy consumption and non-toxic reagents with high yields of concentrated nanoparticles in aqueous solution obtained in less than four minutes. The nanoparticles so obtained are highly monodispersed, thus avoiding the use of further time consuming and cumbersome purification procedures. In addition, the capping agent is readily exchanged by thiols, providing a facile way of functionalization for biological applications.