7β,27-dihydroxycholesterol is a selective endogenous agonist for RORγ and RORγt, activating receptor signaling and enhancing IL-17A production in both human Th17 cells and mouse γδ T cells in vitro and in vivo. This oxysterol is a valuable research tool for studying Th17 biology, autoimmunity, and RORγt-related transcriptional regulation.