It has been hypothesized that certain viral infections directly activate a transcription factor (s) which is responsible for the activation of genes encoding type I interferons (IFNs) and interferon‐stimulated genes (ISGs) via interferon regulatory factor (IRF) motifs present in their respective promoters. These events trigger the activation of defense machinery against viruses. Here we demonstrate that IRF‐3 transmits a virus‐induced signal from the cytoplasm to the nucleus. In unstimulated cells, IRF‐3 is present in its inactive form, restricted to the cytoplasm due to a continuous nuclear export mediated by nuclear export signal, and it exhibits few DNA‐binding properties. Virus infection but not IFN treatment induces phosphorylation of IRF‐3 on specific serine residues, thereby allowing it to complex with the co‐activator CBP/p300 with simultaneous nuclear translocation and its specific DNA binding. We also show that a dominant‐negative mutant of IRF‐3 could inhibit virus‐induced activation of chromosomal type I IFN genes and ISGs. These findings suggest that IRF‐3 plays an important role in the virus‐inducible primary activation of type I IFN and IFN‐responsive genes.