Since it was implicated in a number of neurodegenerative conditions, such as Alzheimer disease, formation of beta-sheet-rich protein fibrils (amyloids) has been drawing a lot of attention. One of elusive aspects of amyloidogenesis concerns the mechanisms of specific binding of molecules such as Congo red, or Thioflavin T by amyloid fibrils. A comprehensive understanding of these docking interactions is needed, however, for the sake of furthering biochemical studies and developing molecular, pharmacological strategies preventing proliferation of amyloids in vivo. Through the application of circular dichroism, here we show that upon binding to insulin fibrils, a twisted conformation is enforced in molecules of Thioflavin T, manifested in a strong negative Cotton effect around 450 nm, which is supported by density functional theory-based calculations. This finding may lead to circular dichroism of Thioflavin T becoming a new diagnostic technique for protein fibrils, complementary to fluorescence spectroscopy.