Herein we describe a protocol that generates Au icosahedra in high yields by simply mixing aqueous solutions of HAuCl(4) and N-vinyl pyrrolidone. Our mechanistic study reveals that water plays an important role in this synthesis: as a nucleophile, it attacks the gold-vinyl complex, leading to the production of an alcohol-based Au(I) intermediate. This intermediate then undergoes a redox reaction in which Au(I) is reduced to Au(0), leading to the formation of Au atoms and then Au icosahedra of about 18 nm in size at a yield of 94 %, together with a carboxylic acid in the final product. This new protocol has also been employed to prepare multiply twinned nanoparticles of Ag (15-20 nm in size), spherical aggregates (25-30 nm in size) of Pd nanoparticles, and very small nanoparticles of Pt (2 nm in size). Since no organic solvent, surfactant, or polymer stabilizer is needed for all these syntheses, this protocol may provide a simple, versatile, and environmentally benign route to noble-metal nanoparticles having various compositions and morphologies.