This paper is focused on the use of oligosaccharide-modified hyperbranched poly(ethyleneimines) (PEI) as reducing and stabilizing agent for the formation of gold nanoparticles. The results show that the secondary amino groups of the PEI as linear units are responsible for the reduction process, and the primary amino groups as terminal units are of relevance for the particle stabilization. With regard to the final size and shape of the gold nanoparticles formed, the amount and type (maltose or maltotriose) of oligosaccharide units and structural parameters of the PEI samples are of importance. The smallest particle size of about 2 nm is obtained from a maltose-modified PEI with an excess of linear units. The size and shape of the polymer-stabilized nanoparticles can be further tuned by changing the solute concentration, the time of heating, as well as the pH value.