A novel thiol-reactive fluorescence probe based on the 1,8-naphthalic anhydride fluorophore was designed and synthesized that selectively detected thiols over other relevant biological species by the turning on of its fluorescence through a Michael addition reaction. In aqueous buffer solutions at physiological pH, the probe exhibited 4-fold enhancement in fluorescence intensity by a Michael addition of Cys, Hey, and GSH to the maleimide appended to a naphthalic. Cell permeability and live cell imaging of thiols are also demonstrated.