The osmotically inducible protein OsmC, like its better-characterized homolog, the organic hydroperoxide protein Ohr, is involved in defense against oxidative stress caused by exposure to organic hydroperoxides. The crystal structure of Escherichia coli OsmC reported here reveals that the protein is a tightly folded domain-swapped dimer with two active sites located at the monomer interface on opposite sides of the molecule. We demonstrate that OsmC preferentially metabolizes organic hydroperoxides over inorganic hydrogen peroxide. On the basis of structural and enzymatic similarities, we propose that the OsmC catalytic mechanism is analogous to that of the Ohr proteins and of the structurally unrelated peroxiredoxins, directly using highly reactive cysteine thiol groups to elicit hydroperoxide reduction.