Updated project metadata. Mitochondrial oxidative phosphorylation (OXPHOS) makes ATP and supports biosynthesis during proliferation, but its role in non-proliferating cells, beyond ATP production, is less understood. Here we show that OXPHOS protects quiescent (but not proliferating) cells from oxidative stress. Using in vivo models of OXPHOS deficiency (whole body and endothelium-specific) we show that OXPHOS mediated resistance to ROS (i) maintains selectivity of ROS-based anticancer therapy by protecting normal tissues during treatment, and in quiescent endothelium (ii) ameliorates systemic LPS-induced inflammation and (iii) attenuates symptoms of the inflammatory bowel disease. ROS-resistance provided by OXPHOS is independent of its role in biosynthesis or NADH recycling. Instead, in quiescent cells OXPHOS constitutively generates low levels of endogenous ROS that support basal autophagic flux and protect from exogenous ROS challenge. Hence, during evolution cells acquired mitochondria to profit from oxidative metabolism, but also built in an OXPHOS-dependent mechanism to guard against the resulting oxidative stress.