Follicular helper T (T_FH) 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_FH cell survival remains unclear. Here we report that T_FH 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_FH cell survival. The deletion of GPX4 in T cells selectively abrogated T_FH cells and germinal center responses in immunized mice. Selenium supplementation enhanced GPX4 expression in T cells, increased T_FH 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_FH homeostasis, which can be targeted to enhance T_FH cell function in infection and following vaccination.