We report the synthesis of polygonal gold nanoparticles (GNPs) by an in situ reduction technique using ferric ammonium citrate as reducing agent in absence of any surfactant or polymeric template. Transmission electron microscopic analysis and selected area electron diffraction patterns confirmed the formation of well-crystalline polygonal GNPs grown preferentially along the (111) direction, which is consistent with the results of X-ray diffractometry analysis. The results of control experiments of HAuCl4 with tri-ammonium citrate in presence of different externally added metal ions like Fe3+, Ni2+, Cu2+, Zn2+, and Al3+ suggested the ion-induced growth mechanism in the formation of polygonal GNPs. The purified polygonal GNPs were then successfully used as catalyst in the borohydride reduction of three isomeric nitrophenols and also in the aerobic oxidation of different D-hexoses (e.g., D-glucose, D-mannose, D-fructose). The catalytic activity of these polygonal GNPs is higher by a factor of 300–1000, depending on the GNP's sample type, in nitrophenol reduction compared to that of spherical GNPs. Similar activity enhancement was also observed in the aerobic oxidation of different D-hexoses. These polygonal GNPs catalyst are very stable and could be reused several times in the borohydride reduction of nitrophenols without much losing in their virgin catalytic activity.