Plasmacytoid dendritic cells (pDCs) are a specialized subset of DCs capable of rapidly producing massive amounts of type I IFN (IFN-I) in response to viral infections. The mechanism regulating rapid production of IFN-I after pDCs are exposed to viral nucleic acids remains elusive. Here, we show that the transcription factor Blimp-1 is expressed in pDCs and is upregulated in a Rac-1-dependent manner after short-term exposure to TLR7 and TLR9 ligands. Conditional or inducible deletion of the Prdm1 gene encoding Blimp-1 in pDCs impaired production of IFN-I, but not other cytokines, upon treatment with CpG-A or various viral infections. Accordingly, mice lacking Blimp-1 in DCs failed to produce IFN-I after CpG-A treatment and did not mount antiviral responses following flavivirus infection. Mechanistically, we found the activation of IKKα and IRF7 was impaired, but canonical NF-κB p65 and p50 were activated normally in CpG-A stimulated Blimp-1-deficient pDCs. Together, we identify a Blimp-1-dependent pathway that rapidly facilitates IFN-I production in pDCs.