The transcriptional control of CD1d-restricted NKT cell development has remained elusive. We report that PLZF (promyelocytic leukemia zinc finger, Zbtb16), a member of the BTB/POZ-ZF family of transcription factors that includes the CD4-lineage-specific c-Krox (Th-POK), is exquisitely specific to CD1d-restricted NKT cells and human MR1-specific MAIT cells. PLZF was induced immediately after positive selection of NKT cell precursors, and PLZF-deficient NKT cells failed to undergo the intrathymic expansion and effector differentiation that characterize their lineage. Instead, they preserved a naive phenotype and were directed to lymph nodes. Conversely, transgenic expression of PLZF induced CD4⁺ thymocytes to acquire effector differentiation and migrate to nonlymphoid tissues. We suggest that PLZF is a transcriptional signature of NKT cells that directs their innate-like effector differentiation during thymic development.