The femtosecond fluorescence upconversion technique is used to study the effect of viscosity on the excited state relaxation dynamics of an amyloid fibril sensor, thioflavin-T, in different solvent media. The excited state decay in all of the solvents is seen to be dependent on the emission wavelength. From the constructed time-resolved emission spectra, it is seen that the present system shows dynamic Stokes' shift as well as an appreciable increase in the spectral width with time. These temporal spectral characteristics of time-resolved emission spectra have been assigned to the formation of a new emissive species from the locally excited state of the thioflavin-T molecule. The formation of the new emissive state from the locally excited state is also supported by the fact that an iso-emissive point appears in the time-resolved area normalized emission spectra. From the detailed study on the excited state dynamics of thioflavin-T as a function of solvent viscosity, it is concluded that the new emissive state is formed due to the twisting around the central C-C single bond in the excited state of thioflavin-T. The formation rate of the twisted emissive state from the locally excited state is found to be nicely correlated with the viscosity of the medium.