Hyperbranched poly(ethylenimine) (PEI) with amino groups or carbohydrate terminal groups have been used as support materials for metal nanoparticles (i.e., Cu, Ag, Au, and Pt) in water. Various parameters have been optimized, such as pH, concentration of the polymer in solution, and [metal ions]/ [polymer] ratio in order to obtain stable metal nanoparticles with a narrow size distribution. TEM measurements revealed that particles with a diameter as low as 1.4 nm were obtained. An increase of stability was obtained after functionalization of PEI with glycidol, gluconolactone, or lactobionic acid. In the case of Pt the catalytical activity of the corresponding nanoparticles was evidenced.