We report a new function for Escherichia coli DsbC, a protein best known for disulfide bond isomerization in the periplasm. We found that DsbC regulates the redox state of the single cysteine of the l-arabinose-binding protein AraF. This cysteine, which can be oxidized to a sulfenic acid, mediates the formation of a disulfide-linked homodimer under oxidative stress conditions, preventing l-arabinose binding. DsbC, unlike the homologous protein DsbG, reduces the intermolecular disulfide, restoring AraF binding properties. Thus, our results reveal a new link between oxidative protein folding and the defense mechanisms against oxidative stress.