Effects of solvent viscosity on the thermal isomerization properties of a spiropyran derivative have been studied in glycerol, ethylene glycol, 1,4-butanediol, and ionic liquid solutions. Thermal isomerization of the colorless spiroyclic (SP) form to the colored merocyanine (MC) form is enhanced with an increase in the concentrations of viscous solvents in solution. Equilibrium absorption analysis revealed that the enhanced SP -> MC isomerization in viscous media is due to the strong solvent-solvent interaction, which suppresses the ordering of solvent molecules around the MC form. This results in a positive entropy change for isomerization and, hence, promotes entropy-driven isomerization. Kinetic absorption analysis revealed that the solvent viscosity scarcely affects the thermal activation process for isomerization, where the activation enthalpy and entropy parameters are solely affected by the solvent polarity.