A process for producing polymer-supported metal nanoparticles involves confinement of metal nanoparticles in polymeric nanotubes or nanosheets in an aqueous environment using hydrophobic reactants. Metal nanoparticles supported in the polymeric nanotubes or nanosheets are substantially monodisperse and have an average particle size of 4 nm or less. The polymer-supported metal nanoparticles are useful in fuel cells, sensors, bioanalysis, biological labeling or semi-conductors, especially as catalysts.