We report an organo-paramagnetic switch consisting of a linked bis(viologen) dication diradical that can be cycled reversibly between diamagnetic and paramagnetic states via noncovalent guest–host chemistry with cucurbit[7]uril (CB[7]) in room-temperature water. Computations suggest that the nature of the interaction between the viologen cation radical units is that of a pi dimer (pimer). Molecules with switchable magnetic properties have possible applications in spintronics, data storage devices, chemical sensors, building blocks for materials with switchable bulk magnetic properties, as well as magnetic resonance probes for biological applications.