Poxviruses are DNA viruses that express numerous proteins to subvert the host immune response. Vaccinia virus protein K7 adopts a Bcl-2 fold and displays structural and functional similarities to Toll-like receptor antagonist A52. Both proteins interact with IRAK2 and TRAF6 and suppress TLR-dependent NF-κB activation. However, unlike A52, K7 also forms a complex with RNA helicase DDX3 and antagonizes interferon-β promoter induction. We have narrowed the K7 binding site to an N-terminal peptide motif of DDX3 ahead of its core RNA-helicase domains. The crystal structure of full-length K7 in complex with the DDX3 peptide reveals a thumblike projection of tandem phenalyalanine residues of DDX3 into a deep hydrophobic cleft. Mutagenesis of these phenylalanines abolishes the effects of DDX3 on interferon-β promoter induction. The structure of K7-DDX3 reveals a novel binding mode by a viral Bcl-2 protein that antagonizes a key pathway in innate immunity.