We have recently reported that Ca2+/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) is inactivated by reactive sulfur species via polysulfidation of the active-site Cys residue. Here, we show that hydrogen peroxide (H2O2) limit CaMKIV activity at the same active-site Cys residue through oxidation and downstream signaling in cells. CaMKIV is phosphorylated at Thr196 by its upstream CaMK kinase (CaMKK), which induces its full activity. In vitro incubation of CaMKIV with H2O2 resulted in reversible inhibition of CaMKK-induced phospho-Thr196 and the consequent inactivation of CaMKIV. In contrast, mutated CaMKIV (C198V) was refractory to the H2O2-induced enzyme inhibition. In transfected cells expressing CaMKIV, Ca2+ ionophore-induced CaMKIV phosphorylation at Thr196 was decreased upon treatment with H2O2, whereas cells expressing mutant CaMKIV (C198V) were resistant to H2O2 treatment. Modification of free thiol with N-ethylmaleimide revealed that Cys198 in CaMKIV is a target for S-oxidation. Additionally, the Ca2+ influx-induced phospho-Thr196 of endogenous CaMKIV was also inhibited upon treatment with H2O2 in Jurkat T-lymphocytes and cerebellar granule cells. Phosphorylation of cyclic AMP response element-binding protein (CREB) at Ser133, which is downstream of CaMKIV, was also decreased upon treatment with H2O2. Thus, our results indicate that oxidation stress regulates cellular function by decreasing the activity of CaMKIV through Cys198 oxidation.