Anti-cancer drugs often increase reactive oxygen species (ROS) and causeDNAdamage. Here, we highlight a new cross talk between chronic oxidative stress and the histone variant H2AX, a key player inDNArepair. We observe that persistent accumulation ofROS, due to a deficient JunD-/Nrf2-antioxidant response, reduces H2AXprotein levels. This effect is mediated by an enhanced interaction of H2AXwith the E3 ubiquitin ligaseRNF168, which is associated with H2AXpoly-ubiquitination and promotes its degradation by the proteasome.ROS-mediated H2AXdecrease plays a crucial role in chemosensitivity. Indeed, cycles of chemotherapy that sustainably increaseROSreduce H2AXprotein levels in Triple-Negative breast cancer (TNBC) patients. H2AXdecrease by such treatment is associated with an impairedNRF2-antioxidant response and is indicative of the therapeutic efficiency and survival ofTNBCpatients. Thus, our data describe a novelROS-mediated regulation of H2AXturnover, which provides new insights into genetic instability and treatment efficacy inTNBCpatients.