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)is 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), nanoparticles are best considered as ligand-protected superatoms.