The absorption and fluorescence spectra of a new styryl derivative of thioflavin T tosylate 2-{(1E,3E)-4- [4-(dimethylamino)-2,6-dimethylphenyl]buta-1,3-dien-1-yl}- 3-ethyl-1,3-benzothiazolium-3 (Th-C23) in solvents of different polarity and viscosity, as well as when it is incorporated into the structure of amyloid fibrils and bovine serum albumin are investigated. A characteristic feature of the dye is an extremely low fluorescence quantum yield in low-viscosity solvents (10–4 in water), which, however, increases significantly in viscous solutions and when incorporated into the structure of proteins or amyloid fibrils. In the latter case, the quantum yield increases by 8·103 times. Based on experimental studies and quantum chemical calculations, it is shown that Th-C23 exhibits molecular rotor properties. An increase in the fluorescence quantum yield in viscous solutions and upon incorporation into biopolymers is the result of limiting the torsion rotation of molecular fragments, leading to fluorescence quenching. The long-wavelength position of the absorption spectrum and the fluorescence spectrum of the new dye in the red region of the spectrum (520 and 600 nm) makes it possible to use it as a fluorescent marker sensitive to the viscosity (hardness) of the microenvironment not only in vitro, but also in vivo.