A thiocoumarin-based selective fluorescent probe (TCP) for Hg2+ ions has been synthesized and characterized. Addition of Hg2+ ions to a solution of the probe can give a rise of OFF-ON fluorescence due to the Hg2+-triggered transformation of thiocoumarin to a coumarin derivative. Other competing ions including Pb2+, Cd2+, Cr3+, zri2+, Fe2+, Co3+, Ne+, Ca2+, Mg2+, K+ and Na+ ions do not induce any distinct fluorescence changes, indicating that the probe can selectively detect Hg2+ ions in aqueous solution.