IRE1α regulates ferroptosis sensitivity by controlling glutathione synthesis. IRE1α, an endoplasmic reticulum (ER) resident protein involved in the unfolded protein response (UPR), determines cellular sensitivity to ferroptosis. Depletion of IRE1α increases resistance to ferroptosis, while enhanced IRE1α expression promotes sensitivity. Mechanistically, IRE1α's endoribonuclease activity cleaves and down-regulates the mRNA of key glutathione biosynthesis regulators, glutamate-cysteine ligase catalytic subunit (GCLC) and solute carrier family 7 member 11 (SLC7A11). This activity is independent of IRE1α's role in the UPR and is evolutionarily conserved. Genetic deficiency and pharmacological inhibition of IRE1α reduce ferroptosis and renal ischemia-reperfusion injury in mice. IRE1α negatively regulates cellular glutathione availability by suppressing GCLC expression through its RIDD activity. IRE1α also suppresses SLC7A11 expression, which is critical for cystine uptake. In C. elegans, IRE1α deficiency increases resistance to ferroptosis induced by dietary polyunsaturated fatty acid (DGLA). Pharmacological inhibition of IRE1α reduces ferroptosis and alleviates ischemic kidney damage. These findings reveal a previously unknown role of IRE1α in regulating ferroptosis and suggest that inhibition of IRE1α could be a promising therapeutic strategy for ferroptosis-associated pathological conditions.IRE1α regulates ferroptosis sensitivity by controlling glutathione synthesis. IRE1α, an endoplasmic reticulum (ER) resident protein involved in the unfolded protein response (UPR), determines cellular sensitivity to ferroptosis. Depletion of IRE1α increases resistance to ferroptosis, while enhanced IRE1α expression promotes sensitivity. Mechanistically, IRE1α's endoribonuclease activity cleaves and down-regulates the mRNA of key glutathione biosynthesis regulators, glutamate-cysteine ligase catalytic subunit (GCLC) and solute carrier family 7 member 11 (SLC7A11). This activity is independent of IRE1α's role in the UPR and is evolutionarily conserved. Genetic deficiency and pharmacological inhibition of IRE1α reduce ferroptosis and renal ischemia-reperfusion injury in mice. IRE1α negatively regulates cellular glutathione availability by suppressing GCLC expression through its RIDD activity. IRE1α also suppresses SLC7A11 expression, which is critical for cystine uptake. In C. elegans, IRE1α deficiency increases resistance to ferroptosis induced by dietary polyunsaturated fatty acid (DGLA). Pharmacological inhibition of IRE1α reduces ferroptosis and alleviates ischemic kidney damage. These findings reveal a previously unknown role of IRE1α in regulating ferroptosis and suggest that inhibition of IRE1α could be a promising therapeutic strategy for ferroptosis-associated pathological conditions.