氧化逆境（英语：oxidative stress）为机体活性氧成分与抗氧化系统之间平衡失调引起的一系列适应性的反应。 干扰细胞正常的氧化还原状态, 会制造出过氧化物与自由基导致毒性作用, 因此损害细胞的蛋白质, 脂类, 和DNA。源自氧化代谢的氧化应激, 会导致基底损害以及DNA链断裂。

发生在人类的氧化应激, 被认为是造成亚斯伯格症候群, 自闭症, 阿兹海默症, 帕金森氏症, 注意力缺陷过动症, 动脉粥样硬化, 心脏衰竭, 及癌症等的成因。 然而, 活性氧也有它的益处, 免疫系统可利用活性氧攻击并杀死病原。 短期的氧化应激在防止老化上, 也提供了重要的步骤, 称做毒物兴奋效应。

Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal redox state of cells can cause toxic effects through the production of peroxides and free radicals that damage all components of the cell, including proteins, lipids, and DNA. Oxidative stress from oxidative metabolism causes base damage, as well as strand breaks in DNA. Base damage is mostly indirect and caused by reactive oxygen species (ROS) generated, e.g. O₂ (superoxide radical), OH (hydroxyl radical) and H₂O₂ (hydrogen peroxide). Further, some reactive oxidative species act as cellular messengers in redox signaling. Thus, oxidative stress can cause disruptions in normal mechanisms of cellular signaling.