Supported noble metal nanoparticles (NMNPs) are appealing for energy and environment catalysis. To facilitate the loading of NMNPs, in situ reduction of Mn+ on the support with extra reductants/surfactants is adopted, but typically results in aggregated NMNPs with uneven size distributions or blocked active sites of the NMNPs. Herein, the use of cobalt layered double hydroxide (Co-LDH) is proposed as both support and reductant for the preparation of supported NMNPs with ultrasmall sizes and even distributions. The resultant Co-LDH-supported NMNPs exhibit excellent catalytic performance and stability. For example, Ir/Co-LDH displays a low overpotential of 188mV (10mAcm-2 ) for electrocatalytic oxygen evolution reaction and a long-term stability over 100h (100mAcm-2 ) in overall water splitting. Ru/Co-LDH can achieve a 4-nitrophenol reduction with high rate of 0.36min-1 and S2- detection with low limit of detection (LOD) of 0.34µm. Overall, this work provides a green and effective strategy to fabricate supported NMNPs with greatly improved catalytic performances.