Water-dispersible PEGylated nanoparticles (NPs) presenting amine-reactive conjugation sites at their surfaces were synthesized and their ability to react with amines was demonstrated. An amphiphilic block copolymer bearing an N-succinimidyl ester at its water-soluble end was synthesized by the consecutive controlled radical polymerization of poly(ethylene glycol) methacrylate and styrene from a functional halide initiator. After purification of the copolymer, NPs of approximately 40 nm were obtained by a self-assembly process in water. The reactivity of the NPs was evidenced by reacting them with primary amines, including a fluorescent dye. The activated ester remained stable throughout all synthetic steps and a nearly quantitative coupling efficiency was obtained.