The major bifurcation in dendritic cell (DC) differentiation occurs when a common progenitor becomes either a conventional DC (cDC) or plasmacytoid DC (pDC). Despite the importance of this lineage decision, its genetic basis has not been determined. 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. Indeed, 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 regulation of the little studied transcriptional regulator DC-SCRIPT. PU.1 promoted DC-SCRIPT expression, which we identify as essential in determining cDC versus pDC cell fate and subsequent cDC diversification. We reveal a novel transcriptional node controlling cDC identity regulated by PU.1 and DC-SCRIPT.