Abstract Monodisperse spherical polyacrylonitrile (PAN) nanoparticles were successfully prepared for the first time by dispersion polymerization of acrylonitrile (AN) in water using well-defined poly(N-vinyl pyrrolidone) (PVP) that was end-capped by a cobalt(II) acetylacetonate (Co(acac)2) complex (PVP-Co(acac)2) as both a macroinitiator and a colloidal stabilizer. The well-defined PVP-Co(acac)2 (Mn = 14,000 g/mol, PDI = 1.25) was synthesized by the bulk cobalt-mediated radical polymerization of N-vinyl pyrrolidone at 20 °C using 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) as an initiator and Co(acac)2 as a regulating agent. The PVP macroradicals generated at 30 °C by the homolytic cleavage of the C–Co bonds in PVP-Co(acac)2 initiated the dispersion polymerization of AN, as well as successfully stabilized the growing PAN particles. The average diameters of PAN nanoparticles synthesized with 20, 30, 40, and 50 wt% of PVP-Co(acac)2 at 30 °C for 24 h were 263.5, 163.1, 157.3, and 143.5 nm, respectively. The PAN nanoparticles had a slightly crumpled spherical appearance, and the degree of crystallinity of the PAN nanoparticles prepared using 30 wt% of PVP-Co(acac)2 was 31.2%. The mol% of VP units in the PAN nanoparticles was about 6 mol%, and the PVP chains were present on the surface of the PAN nanoparticles as a stabilizing layer. The PVP hairy chains could successfully stabilize very small Ag nanoparticles on the surface of the PAN nanoparticles.