Preeclampsia (PE) is a dangerous complication of pregnancy, especially when it presents at <34 wk of gestation (PE < 34 wk). It is a major cause of maternal and fetal morbidity and mortality and also increases the risk of cardiometabolic diseases in later life for both mother and offspring. Placental oxidative stress induced by defective placentation sits at the epicenter of the pathophysiology. The placenta is susceptible to activation of the unfolded protein response (UPR), and we hypothesized this may affect mitochondrial function. We first examined mitochondrial respiration before investigating evidence of mitochondrial UPR (UPRmt) in placentas of PE < 34 wk patients. Reduced placental oxidative phosphorylation (OXPHOS) capacity measured in situ was observed despite no change in protein or mRNA levels of electron transport chain complexes. These results were fully recapitulated by subjecting trophoblast cells to repetitive hypoxia-reoxygenation and were associated with activation of a noncanonical UPRmt pathway; the quality-control protease CLPP, central to UPRmt signal transduction, was reduced, while the cochaperone, TID1, was increased. Transcriptional factor ATF5, which regulates expression of key UPRmt genes including HSP60 and GRP75, showed no nuclear translocation. Induction of the UPRmt with methacycline reduced OXPHOS capacity, while silencing CLPP was sufficient to reduce OXPHOS capacity, membrane potential, and promoted mitochondrial fission. CLPP was negatively regulated by the PERK-eIF2α arm of the endoplasmic reticulum UPR pathway, independent of ATF4. Similar changes in the UPRmt pathway were observed in placentas from PE < 34 wk patients. Our results identify UPRmt as a therapeutic target for restoration of placental function in early-onset preeclampsia.