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Abstract Interferons (IFNs) activate JAK‐STAT pathways to induce downstream effector genes for host defense against invaded pathogens and tumors. Here both type I (β) and II (γ) IFNs are shown that can activate the transcription factor IRF3 in parallel with STAT1. IRF3‐deficiency impairs transcription of a subset of downstream effector genes induced by IFN‐β and IFN‐γ. Mechanistically, IFN‐induced activation of IRF3 is dependent on the cGAS‐STING‐TBK1 axis. Both IFN‐β and IFN‐γ cause mitochondrial DNA release into the cytosol. In addition, IFNs induce JAK1‐mediated tyrosine phosphorylation of cGAS at Y214/Y215, which is essential for its DNA binding activity and signaling. Furthermore, deficiency of cGAS, STING, or IRF3 impairs IFN‐β‐ or IFN‐γ‐mediated antiviral and antitumor activities. The findings reveal a novel IRF3 activation pathway parallel with the canonical STAT1/2 activation pathways triggered by IFNs and provide an explanation for the pleiotropic roles of the cGAS‐STING‐IRF3 axis in host defense.