The recognition of intracellular pathogens and induction of cytokine signalling is tightly regulated at the cellular level, and is a target for manipulation by pathogens and therapeutics alike. As part of a genome-wide screen of cellular proteins required for Hendra virus infection, we identified an uncharacterised protein required for infection by Hendra virus, in addition to a diverse range of RNA viruses, including highly pathogenic avian influenza virus and West Nile virus. Given its association with a broad-spectrum of RNA viruses, we hypothesised that this uncharacterised host protein modulates the type I interferon antiviral immune response. Our subsequent studies have shown that the uncharacterised protein is a negative regulator of the transcription and secretion of interferon (IFN)a/B and the pro-inflammatory cytokine tumor necrosis factor (TNF)α, in response to the viral RNA mimic poly(I:C). Interestingly, the decreased transcription of these cytokines could not be attributed to inhibition of upstream signalling, as nuclear translocation of the transcription factors IRF3 and NFkB was observed in cells with impaired cytokine production. We also observed nuclear localisation of both the endogenous and overexpressed unknown protein in the nucleus. In summary we have identified a novel regulator of the prototypic type I IFN antiviral immune response with implications for antiviral immunity against RNA viruses.