The on–off switching of spin-crossover (SCO) and single-molecule magnetism (SMM) remains highly attractive, especially if it involves dynamic crystal-to-crystal transformation. Herein we report the first molecule, a mononuclear cobalt(II) complex, that exhibits on–off switching between SCO and SMM reversibly during crystal-to-crystal transformation. Subtle structural transformation triggered by a simple dehydration–rehydration process induces significant geometrical changes of the Co^II center and modification of the supramolecular interactions and switches its colour and magnetic properties (dark red/SCO-on/SMM-off ↔ orange/SCO-off/SMM-on). This work suggests that modification of the weak supramolecular interactions could be very effective in achieving switchable materials involving both SCO and SMM properties.