We report reversible switching of paramagnetism in a well-defined gold nanoparticle system consisting of atomically monodisperse nanoparticles containing 25 gold atoms protected by 18 thiolates [abbreviated as Au(25)(SR)(18)]. The magnetism in these nanoparticles can be switched on or off by precisely controlling the charge state of the nanoparticle, that is, the magnetic state of the Au(25)(SR)(18) nanoparticles is charge-neutral while the nonmagnetic state is an anionic form of the particle. Electron paramagnetic resonance (EPR) spectroscopy measurements establish that the magnetic state of the Au(25)(SR)(18) nanoparticles possess one unpaired spin per particle. EPR studies also imply an unusual electronic structure of the Au(25)(SR)(18) nanoparticle. Density functional theory calculations coupled with the experiments successfully explain the origin of the observed magnetism in a Au(25)(SR)(18) nanoparticle as arising from one unpaired spin having distinct P-like character and delocalized among the icosahedral Au(13) core of the particle in the highest occupied molecular orbital. The results suggest that the Au(25)(SR)(18) nanoparticles are best considered as ligand-protected superatoms.