Monocytes regulate host defenses, inflammation and tissue homeostasis. The transcription factor (TF) Interferon Regulatory Factor-8 (IRF8) stimulates monocyte/macrophage differentiation, yet genome-wide understanding of the differentiation program initiated by IRF8 is lacking. By combining chromatin immunoprecipitation-sequencing with gene expression profiling, we show here that during IRF8-dependent monocyte differentiation, IRF8 binding occurs at both promoter-proximal and distal regions together with the TF PU.1 and is associated with gene induction. Many of the promoter-distal IRF8 binding sites show an increase in histone H3 lysine 4 monomethylation, a signature for enhancers. However, about half of IRF8-induced genes were not bound by IRF8, suggesting the involvement of downstream TFs. Analysis of DNA motifs in cis-regulatory elements of these indirect IRF8 target genes predicted that Krüppel-like factor-4 (KLF4), which is essential for Ly6C(+) monocyte development, is one such factor. Indeed, monocyte development in Irf8(-/-) mice is as defective as that in Klf4(-/-) chimeric mice. Moreover, Irf8(-/-) monocyte-dendritic cell progenitors do not express Klf4 mRNA. Introduction of KLF4 into an Irf8(-/-) myeloid progenitor cell line induced a subset of IRF8 target genes and caused partial monocyte differentiation. Taken together, our present results uncover genome-wide behavior of IRF8 and identify an IRF8-KLF4 axis that operates during monocyte differentiation.