Herein we report a facile synthesis of gold nanoparticle/polymer nanocomposite particles through a spontaneous reduction of tetrachloroauric (III) acid and encapsulation of resultant gold nanoparticles using amine-rich polymeric particles in water. The particle consisted of core-shell nanostructure with a dense layer of polyethyleneimine (PEI) shell. The hydrophilic PEI shell acted as a reductant to generate gold nanoparticles (AuNPs), while the particle as a polymer template to in-situ encapsulate and stabilize the resultant AuNPs, giving a stable gold/polymer nanocomposite particle in water. The PEI-based core-shell particles with different degree of softness were found to have little influence on the reduction ability of the gold salt, but considerablely affect the encapsulating capability. Increasing the level of softness of particle core gave higher encapsulation efficiency to the gold nanoparticles. A soft type of core-shell microgel, namely poly(N-isopropyl acrylamide)/polyethyleneimine (PNIPAm/PEI) was further investigated with respect to the reduction rate, encapsulation efficiency, as well as nanocomposite stability and properties. Results indicated that reduction rate of gold salts using the PNIPAm/PEI microgels as the reductant was two orders of magnitude faster than that of the native PEI. Solution pH and amine to gold salt ratio also affected the formation of Au@PNIPAm/PEI composite particles. Our results demonstrate that the use of polymeric amine-based particles is a simple and green synthesis of Au@polymer nanocomposite particles in aqueous system which is free from organic solvent, reducing and stabilizing agents. (C) 2015 Elsevier Ltd. All rights reserved.