The mitochondrial unfolded protein response (UPRmt) is critical for protecting mitochondria against proteotoxic stress. Current UPRmt models propose that mitochondrial defects are detected by cytosolic surveillance mechanisms after release of damage into the cytosol. However, these findings are based on models that induce rapid and severe mitochondrial dysfunction, and thus their mode of toxicity contrasts sharply from physiologically occurring mitochondrial damage, e.g. the gradual accumulation of reactive oxygen species (ROS) during age-related decline or chronic respiratory chain defects. Here, we employed a chemogenetic strategy that induces low levels of H2O2 in the mitochondrial matrix to investigate cellular responses to physiologically relevant levels of mitochondrial dysfunction. We uncover that this mild oxidative stress activates the UPRmt independently of cytosolic signals revealing a so far concealed primary surveillance mechanism within mitochondria. Moreover, we identify the mitochondrial presequence proteases MPP and Oct1 as early molecular targets of ROS: Oxidative stress induces glutathionylation of critical cysteine residues, resulting in diminished proteolytic activity and the accumulation of proteotoxic precursor aggregates in the matrix. These aggregates are detected by intramitochondrial surveillance systems, activating UPRmt signaling. Our findings uncover the primary response to mitochondrial dysfunction, and highlight the organelle's capacity for self-surveillance and its ability to initiate early and rapid protective signaling in the face of mitochondrial dysfunction.