Polyethylenimine-modified montmorillonite (N-MMT) was used to prepare gold nanoparticles, where the montmorillonite (MMT) acted as a solid support to retain the conformation of polyethylenimine (PEI), and the amino groups of PEI were used simultaneously to both complex and reduce the gold ions. From the results of X-ray diffraction, it is apparent that the reduction of gold ions occurs primarily on the MMT surface. In the presence of MMT, the formation of a flattened configuration on the clay instead of stretched-out ethylenimine segments of PEI results in the formation of smaller gold particles. With a higher acidification ratio, the recharging of the MMT surface with positive ammonium ionic sites of PEI is likely to prevent the flocculation of clay and thus facilitate the reduction of gold. The rate of gold reduction with N-MMT is faster at low pH values, this being in contrast to the usual trend observed for the reduction of gold ions. The use of PEI adsorbed onto MMT has been shown to be able to act simultaneously as both a protective template and as a reducing agent, thereby greatly simplifying the process for preparing gold nanoparticles.