A but-3-yn-2-one-based 7-diethylaminocoumarin dye was exploited as a fluorescent probe to specifically detect Cys over Hcy/GSH in pure PBS buffer. The probe itself is nonfluorescent due to the donor-excited photoinduced electron transfer (d-PET) process. The Cys-induced Michael addition–rearrangement cascade reaction leads to an amino-substituted product 3a with strong fluorescence due to inhibiting CC isomerization induced fluorescence quenching by a produced intramolecular N–H⋯O hydrogen bond. The Hcy (or GSH)-induced Michael addition reaction leads to a thiol-substituted product 2b (or 4), which lacks any intramolecular hydrogen-bonding interaction, and thus displays very poor fluorescence due to the efficient CC isomerization induced fluorescence quenching. Even in the presence of Hcy (or GSH), the probe could also detect Cys with the obvious fluorescence enhancement. Assisted by using a laser scanning confocal microscope, we demonstrated that the probe could selectively image Cys in the human renal cell carcinoma 786-0 cells.