Hydrogen sulfide (H2S) is now recognized as an important gaseous transmitter that is involved in a variety of biological processes. Here, we report the design and synthesis of a luminescent lanthanide biosensor for H2S, LP2-Cu(II)-Ln(III), a heterobinudear metal complex that uses Cu(II) decomplexation to control milli-second-scale-lifetime-Tb(III)- or Eu(III)-emission intensity. LP2-Cu(II)-Ln(III) responded rapidly, selectively, and with high sensitivity to aqueous H2S. The probe's potential for biological applications was verified by measuring the H2S generated by the slow releasing chemical-sulfide-donor GYY4147, by cystathionine gamma-lyase (CSE), and by Na2S-stimulated HeLa cells.