Dendritic cells (DCs) are can be broadly divided into conventional (cDC) and plasmacytoid (pDC) subsets. Despite the importance of this lineage diversity, its genetic basis is not fully understood. We found that conditional ablation of the Ets-family transcription factor PU.1 in DC-restricted progenitors led to increased pDC production at the expense of cDCs. PU.1 controlled many of the cardinal functions of DCs, such as antigen presentation by cDCs and type I interferon production by pDCs. Conditional ablation of PU.1 de-repressed the pDC transcriptional signature in cDCs. The combination of genome-wide mapping of PU.1 binding and gene expression analysis revealed a key role for PU.1 in maintaining cDC identity through the induction of the transcriptional regulator DC-SCRIPT. PU.1 activated DC-SCRIPT expression, which in turn promoted cDC formation, particularly of cDC1s, and repressed pDC development. Thus, cDC identity is regulated by a transcriptional node requiring PU.1 and DC-SCRIPT. Display omitted •PU.1 is required for conventional dendritic cell (cDC) formation and function•PU.1 represses plasmacytoid DC differentiation•DC-SCRIPT (Zfp366) is a key transcriptional target of PU.1•DC-SCRIPT is specifically expressed in cDCs and promotes cDC1 differentiation Dendritic cells can be divided into multiple functional subsets. Chopin et al. show that the transcription factor PU.1 promotes development of conventional dendritic cells while suppressing plasmacytoid dendritic cell formation. PU.1 functions by activating the expression of DC-SCRIPT, a key regulator of conventional dendritic cell differentiation.