I3MT-3 (HMPSNE) is a potent, selective, and cell-membrane permeable inhibitor of 3-Mercaptopyruvate sulfurtransferase (3MST) (IC50=2.7 μM). I3MT-3 is inactive for other H2S/sulfane sulfur-producing enzymes. I3MT-3 targets a persulfurated cysteine residue located in the active site of 3MST.