IFN consensus sequence binding protein (ICSBP/IFN regulatory factor 8) is a hematopoietic cell-specific transcription factor essential for the generation of CD8α+ dendritic cells (DCs). We found that ICSBP−/− mice lack B220+ CD11b− plasmacytoid DCs (pDCs) in addition to CD8α+ DCs. Although ICSBP−/− mice have B220− CD11b+ myeloid DCs (mDCs), they fail to mature upon Toll-like receptor signaling. Accordingly, ICSBP−/− bone marrow progenitor cells were Tefective in generating pDCs in the fms-like tyrosine kinase 3 ligand-based culture system and mDCs generated in this system were defective in maturation. We demonstrate that introduction of ICSBP rescues the development of pDCs from−/− bone marrow progenitors. ICSBP also restored the ability of both pDCs and mDCs to mature after Toll-like receptor signals. ICSBP-restored DCs produced IFN-α and IL-12p40 in a DC subset-selective manner with the amounts comparable to those by+/+ DCs. Together, ICSBP is essential for early pDC development and final maturation of both pDCs and mDCs.