This work focuses on the synthesis method of Au nanoparticles protected by a well-defined polymer monolayer. Nanosized, spherical gold clusters coated with poly(N-isopropylacrylamide) (PNIPA) grafts were prepared by controlled radical polymerization. The polymerization of N-isopropylacrylamide was initiated from the surface of a gold nanoparticle modified with 4-cyanopentanoic acid dithiobenzoate for a reversible-addition-fragmentation chain-transfer polymerization. The number mean diameter of the Au core was 3.2 nm as observed by high-resolution transmission electron microscopy. The molar mass of the PNIPA ligand was 21000 g/mol by gel permeation chromatography. The changes in the surface plasmon of gold were investigated in different media, and as functions of particle concentration, as well as of temperature in aqueous solutions. The particles were soluble at least slightly in water, forming aggregates. The area and the maximum wavelength of the plasmon band in water decreased with dilution and increasing temperature. During the collapse of PNIPA ligands the surroundings of the gold surface change from hydrophilic to hydrophobic.