Ferroptosis is a form of regulated cell death triggered by iron-dependent lipid hydroperoxides. Selenoprotein glutathione peroxidase 4 (GPX4) suppresses ferroptosis by detoxifying lipid hydroperoxides. Sec, the genetically encoded 21st amino acid, is biosynthesized from a reactive selenium donor on its cognate tRNA[Ser]Sec. Intracellular selenium must be delivered safely and efficiently by a carrier protein due to its high reactivity and low concentration. This study identified peroxiredoxin 6 (PRDX6) as a novel selenoprotein synthesis factor. Loss of PRDX6 decreases selenoprotein expression and induces ferroptosis by reducing GPX4 levels. Mechanistically, PRDX6 increases the efficiency of intracellular selenium utilization by transferring selenium between proteins within the selenocysteyl-tRNA[Ser]Sec synthesis machinery, leading to efficient synthesis of selenocysteyl-tRNA[Ser]Sec. These findings highlight previously unidentified selenium metabolic systems and provide new insights into ferroptosis. Additionally, high expression of PRDX6 is associated with poor prognosis in various cancers, suggesting that targeting PRDX6 may be an effective strategy for sensitizing cancer cells to iron-triggered ferroptosis.Ferroptosis is a form of regulated cell death triggered by iron-dependent lipid hydroperoxides. Selenoprotein glutathione peroxidase 4 (GPX4) suppresses ferroptosis by detoxifying lipid hydroperoxides. Sec, the genetically encoded 21st amino acid, is biosynthesized from a reactive selenium donor on its cognate tRNA[Ser]Sec. Intracellular selenium must be delivered safely and efficiently by a carrier protein due to its high reactivity and low concentration. This study identified peroxiredoxin 6 (PRDX6) as a novel selenoprotein synthesis factor. Loss of PRDX6 decreases selenoprotein expression and induces ferroptosis by reducing GPX4 levels. Mechanistically, PRDX6 increases the efficiency of intracellular selenium utilization by transferring selenium between proteins within the selenocysteyl-tRNA[Ser]Sec synthesis machinery, leading to efficient synthesis of selenocysteyl-tRNA[Ser]Sec. These findings highlight previously unidentified selenium metabolic systems and provide new insights into ferroptosis. Additionally, high expression of PRDX6 is associated with poor prognosis in various cancers, suggesting that targeting PRDX6 may be an effective strategy for sensitizing cancer cells to iron-triggered ferroptosis.