According to the mitochondrial theory of aging, mitochondrial dysfunction increases intracellular reactive oxidative species production leading to the oxidation of macromolecules and ultimately to cell death. In this study we investigated the role of the mitochondrial methionine sulfoxide reductase B2 in the protection against oxidative stress. We report, for the first time, that overexpression of methionine sulfoxide reductase B2 in mitochondria of acute T-lymphoblastic leukemia MOLT-4 cell line, in which methionine sulfoxide reductase A is missing, markedly protects against hydrogen peroxide-induced oxidative stress by scavenging reactive oxygen species. Addition of hydrogen peroxide provoked a time-gradual increase of intracellular reactive oxygen species leading to a loss in mitochondrial membrane potential and to protein carbonyl accumulation while in methionine sulfoxide reductase B2 overexpressing cells, intracellular reactive oxygen species and protein oxidation remained low with the mitochondrial membrane potential highly maintained. Moreover, in these cells, delayed apoptosis was shown by a decrease in the cleavage of the apoptotic marker poly(ADP-ribose) polymerase-1 and by the lower percentage of annexin-V positive cells in the late and early apoptotic stages. We also provide evidence for the protective mechanism of methionine sulfoxide reductase B2 against protein oxidative damages. Our results emphasize that upon oxidative stress the overexpression of methionine sulfoxide reductase B2 leads to the preservation of mitochondrial integrity by decreasing the intracellular reactive oxygen species build-up through its scavenging role, hence contributing to cell survival and protein maintenance.