Bistable molecules represent a potential miniaturization limit for high-density information technologies. However, molecules exhibit memory effect typically at very low temperatures. This is the case of spin-crossover (SCO) complexes, where the concept of a molecular memory has not been considered due to the fast spin-state interconversion of all previous systems. Breaking this principle, here we report a slow relaxation process found in an SCO iron-triazole polyanionic complex. Multiple experimental evidences confirm the opening of a thermal hysteresis upon solid dilution and even in liquid solution. Density functional theory (DFT) calculations reveal the origin of this unexpected phenomenon to the appearance of an energy barrier that slows down the spin-state relaxation processes at the molecular level. These results show how SCO molecules may store information at room temperature, opening unique opportunities for molecular data storage.