Northern elephant seals deplete blood O₂ stores by up to 90% while diving, exposing the endothelial cell layer in the blood vessels to repeated acute O₂ fluctuations. Similar hypoxia/reoxygenation (H/R) events drive pathological oxidant generation, inflammation, and endothelial dysfunction in many terrestrial mammals, including humans. Seals, however, appear to tolerate inflammation and oxidant stress, though the specific mechanisms implicated here remain unclear. We isolated endothelial cells (ECs) from seal and human placental arteries to study the molecular drivers of hypoxia and oxidative stress tolerance in seal blood vessels. ECs from seals proliferate in adherent culture and express canonical EC markers (PECAM-1, VE-cadherin). Additionally, seal ECs respire, respond to mitochondrial uncoupling and inhibition, and generate oxidants after pharmacological activation of NADPH oxidase. Seal ECs rapidly and robustly activate protective pathways during hypoxia. Seal ECs upregulated HIF-1α protein levels within 15 min of exposure to 1% O₂ (68-fold increase over t=0), while human ECs did not match this magnitude until 1 hour. Beyond 1 hour, HIF-1α levels in human ECs declined rapidly (from 73- to 22-fold by 6 hours) while remaining high (52-fold) in seal ECs. In addition, oxidant generation in response to H/R or pharmacological stimulation with menadione increased by 5-fold in human ECs (p=0.027) but was attenuated in seal ECs (2.5-fold, p=0.18). Furthermore, exposure to lipid peroxides increased lipid peroxidation in human but not seal ECs. Together, our data demonstrate that seal ECs mount a rapid, sustained protective response to H/R, that these cells are adapted to cope with oxidants, and that this is an effective model system in which to study hypoxia and oxidative stress tolerance in diving mammals.