Follicular helper T (T) cells are a specialized subset of CD4 T cells that essentially support germinal center responses where high-affinity and long-lived humoral immunity is generated. The regulation of T cell survival remains unclear. Here we report that T cells show intensified lipid peroxidation and altered mitochondrial morphology, resembling the features of ferroptosis, a form of programmed cell death that is driven by iron-dependent accumulation of lipid peroxidation. Glutathione peroxidase 4 (GPX4) is the major lipid peroxidation scavenger and is necessary for T cell survival. The deletion of GPX4 in T cells selectively abrogated T cells and germinal center responses in immunized mice. Selenium supplementation enhanced GPX4 expression in T cells, increased T cell numbers and promoted antibody responses in immunized mice and young adults after influenza vaccination. Our findings reveal the central role of the selenium-GPX4-ferroptosis axis in regulating T homeostasis, which can be targeted to enhance T cell function in infection and following vaccination.FH