A new pyrene-based chemosensor (1) exhibits excellent selectivity for Fe3+ ions over a wide range of tested metal ions Ag+, Ca2+, Cd2+, Co2+, Cu2+, Fe 2+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+. The binding of Fe 3+ to chemosensor 1 produces an emission band at 507 nm due to the formation of a Py-Py*excimer that is induced by Fe3+-binding. The binding ratio of 1-Fe3+ was determined to be 1:1 from a Job plot. The association constant of 1-Fe3+ complexes was found to be 1.27 × 104 M-1 from a Benesi-Hildebrand plot. In addition, fluorescence microscopy experiments show that 1 can be used as a fluorescent probe for detecting Fe3+ in living cells.