Time-resolved emission techniques were employed to study the nonradiative process of thioflavin-T (ThT) in 1-propanol, 1-butanol, and 1-pentanol as a function of the hydrostatic pressure. Elevated hydrostatic pressure increases the alcohol viscosity, which in turn strongly influences the nonradiative rate of ThT. A diamond-anvil cell was used to increase the pressure up to 2.4 GPa. We found that the nonradiative rate constant, knr, decreases with pressure. We further found a remarkable linear correlation between a decrease in knr (increase in the nonradiative lifetime, Tnr) and an increase in the solvent viscosity. The viscosity was varied by a factor of 1000 and knr was measured at high pressures, at which the nonradiative rate constant of the molecules decreased from (7 ps)-1 to (13 ns)-1, (13 ps)-1 to (17 ns)-1 and (17 ps) -1 to (15 ns)-1 for 1-propanol, 1-butanol, and 1-pentanol, respectively. The viscosity-dependence of knr is explained by the excited-state rotation rate of the two-ring systems, with respect to each other.