Human mitochondrial glutaredoxin 2 (Grx2) is a glutathione-dependent oxidoreductase (active site: Cys-Ser-Tyr-Cys) that facilitates the maintenance of mitochondrial redox homeostasis upon induction of apoptosis by oxidative stress. Here, we have characterized Grx2 as an iron–sulfur center-containing member of the thioredoxin fold protein family. Mössbauer spectroscopy revealed the presence of a four cysteine-coordinated nonoxidizable [2Fe-2S]²⁺ cluster that bridges two Grx2 molecules via two structural Cys residues to form dimeric holo Grx2. Coimmunoprecipitation of radiolabeled iron with Grx2 from human cell lines indicated the presence of the cluster in vivo. The [2Fe-2S]-bridged dimer was enzymatically inactive, but degradation of the cluster and the resulting monomerization of Grx2 activated the protein. Slow degradation under aerobic conditions was prevented by the presence of glutathione, whereas glutathione disulfide as well as one-electron oxidants or reductants promoted monomerization of Grx2. We propose that the iron–sulfur cluster serves as a redox sensor for the activation of Grx2 during conditions of oxidative stress when free radicals are formed and the glutathione pool becomes oxidized.