Interferon regulatory factor 4 (IRF4) and IRF8 regulate B, T, macrophage, and dendritic cell differentiation. They are recruited to cis-regulatory Ets-IRF composite elements by PU.1 or Spi-B. How these IRFs target genes in most T cells is enigmatic given the absence of specific Ets partners. Chromatin immunoprecipitation sequencing in T helper 17 (T_H17) cells reveals that IRF4 targets sequences enriched for activating protein 1 (AP-1)–IRF composite elements (AICEs) that are co-bound by BATF, an AP-1 factor required for T_H17, B, and dendritic cell differentiation. IRF4 and BATF bind cooperatively to structurally divergent AICEs to promote gene activation and T_H17 differentiation. The AICE motif directs assembly of IRF4 or IRF8 with BATF heterodimers and is also used in T_H2, B, and dendritic cells. This genomic regulatory element and cognate factors appear to have evolved to integrate diverse immunomodulatory signals.