Mitochondrial oxidative phosphorylation (OXPHOS) generates ATP, but also has other important roles that are ATP-independent. In proliferating cells OXPHOS is required for pyrimidine synthesis, but in non-proliferating cells its ATP-unrelated functions have not been defined. Here we demonstrate that in non-proliferating post-mitotic cells OXPHOS maintains stress resistance. OXPHOS deficiency leads to suppression of autophagy and increased sensitivity to oxidative stress in vitro and in TFAM knockout mice in vivo, and attenuation of autophagy in OXPHOS-functional background phenocopies the effects of OXPHOS deficiency. Mechanistically, the autophagy/stress response defect is independent of transcriptional control, AMPK/mTOR1/ULK1 signaling or upregulation of NADH. Instead, absence of OXPHOS-derived ROS in OXPHOS-deficient cells leads to excessive activation of ATG4, a redox-sensitive suppressor of autophagy. We propose that maintenance of stress resistance via the ROS-ATG4 axis is an important function of mitochondrial respiration in non-proliferating cells that may have consequences for longevity and cancer therapy.