Protein structures which form fibrils have recently Excitation been shown to absorb light at energies in the near UV range and to exhibit a structure-specific fluorescence in the visible range even in the absence of aromatic amino acids. However, the molecular origin of this phenomenon-has so far remained elusive. Here, we combine ab initio molecular dynamics simulations and fluorescence spectroscopy to demonstrate that these intrinsically fluorescent protein fibrils are permissive to proton transfer across hydrogen bonds which can lower electron excitation energies and thereby decrease the likelihood of energy dissipation associated with: conventional hydrogen bonds. The importance of proton transfer on the intrinsic fluorescence observed in protein fibrils is signified by large reductions in the fluorescence intensity upon either fully protonating, or deprotonating, the fibrils at pH = 0 or 14, respectively. Thus, our results point to the existence of a structure-specific fluorophore that does not require the presence of aromatic residues or multiple bond conjugation that characterize conventional fluorescent systems. The phenomenon may have a wide range of implications in biological systems and in the design of self-assembled functional materials.