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Improved fluorescent detection of amyloid protein aggregates would accelerate research into the biology of amyloid formation, implicated in Alzheimer's and Parkinson's diseases, among other neurodegenerative conditions. We report the development of a fluorescent sensor OPE1 for amyloids, competent for one- and two-photon imaging, with a strong binding-activated increase in emission based on formation of superluminescent J aggregates and dequenching in a hydrophobic environment. OPE1 stains neurofibrillary tangles and beta-amyloid plaques with low background in murine and human brain tissue, and detects a wide variety of in vitro-formed amyloid aggregates. Unlike conventional sensors such as thioflavin T, OPE1 exhibits static quenching in water based on interactions with chromophore-attached ethyl ester moieties as well as a propensity to form highly emissive J-type aggregates. We provide evidence that both these mechanisms are at work in the large increase in fluorescence intensity observed for OPE1 with amyloid fibrils. We hope that these novel mechanisms will lead to the development of improved self-assembling fluorescent probes for amyloid and other relevant analytes.