Saccharomyces cerevisiae mitochondrial glutaredoxin 5 (Grx5) is the archetypical member of a ubiquitous class of monothiol glutaredoxins with a strictly conserved CGFS active-site sequence that has been shown to function in biological [Fe2S2](2+) cluster trafficking. In this work, we show that recombinant S. cerevisiae Grx5 purified aerobically, after prolonged exposure of the cell-free extract to air or after anaerobic reconstitution in the presence of glutathione, predominantly contains a linear [Fe3S4](+) cluster. The excited-state electronic properties and ground-state electronic and vibrational properties of the linear [Fe3S4](+) cluster have been characterized using UV-vis absorption/CD/MCD, EPR, Mossbauer, and resonance Raman spectroscopies. The results reveal a rhombic S = 5/2 linear [Fe3S4](+) cluster with properties similar to those reported for synthetic linear [Fe3S4](+) clusters and the linear [Fe3S4](+) clusters in purple aconitase. Moreover, the results indicate that the Fe-S cluster content previously reported for many monothiol Grxs has been misinterpreted exclusively in terms of [Fe2S2](2+) clusters, rather than linear [Fe3S4](+) clusters or mixtures of linear [Fe3S4](+) and [Fe2S2](2+) clusters. In the absence of GSH, anaerobic reconstitution of Grx5 yields a dimeric form containing one [Fe4S4](2+) cluster that is competent for in vitro activation of apo-aconitase, via intact cluster transfer. The ligation of the linear [Fe3S4](+) and [Fe4S4](2+) clusters in Grx5 has been assessed by spectroscopic, mutational, and analytical studies. Potential roles for monothiol Grx5 in scavenging and recycling linear [Fe3S4](+) clusters released during protein unfolding under oxidative stress conditions and in maturation of [Fe4S4](2+) cluster-containing proteins are discussed in light of these results.