A synthetic strategy has been developed giving facile access to well-defined single-chain polymer nanoparticles (SCNPs) from styrene-, acrylate- and methacrylate-based polymers. Random copolymers (polydispersity indices 1.10–1.15) of methyl (meth)acrylate, benzyl methacrylate or styrene containing protected thiol monomers (xanthate and thioacetate vinyl monomers) were obtained via reversible addition–fragmentation chain transfer (RAFT) polymerization. Through aminolysis of the xanthate and thioacetate moieties, copolymers with free thiol moieties were obtained. The thiol bearing polymers were cross-linked with bifunctional acrylates under mild conditions. Precursor polymer dependent size-reductions between 30 and 90% were verified by gel permeation chromatography (GPC) measurements. Furthermore, the SCNPs were characterized by 1H NMR, atomic force microscopy (AFM) and dynamic light scattering (DLS). Characteristic patterns for SCNPs were observed in the AFM phase mode. Thiol-Michael addition is demonstrated to be a versatile tool, which can easily be employed in the preparation of versatile well-defined functional polymer nanoparticles in the 3–10 nm size range.