PYHIN (IFI200/HIN-200) family members are involved in host antiviral immune responses by detecting viral nucleic acids through HIN domains. Within this family, the function of IFI208, which lacks a HIN domain, remains largely unknown. In this study, we identify IFI208, a member of the PYHIN family, as an interferon stimulated gene (ISG). Upon exposure to viral DNA and RNA, IFI208 inhibits viral replication by increasing type I interferon (IFN) production. Mechanistically, upon infection, IFI208 is translocated to the cytoplasm through the chromosome region maintenance 1 (XPO1) transporter protein, and interacts with melanoma differentiation associated gene 5 (MDA5) to enhance the ability of this protein to recognize double stranded RNA (dsRNA) and promote its oligomerization. Taken together, we identify IFI208 as an interferon-stimulated gene (ISG) that induces dsRNA dependent liquid-liquid phase separation of MDA5. Our results indicate that the XPO1-IFI208-MDA5 axis has an important function in the innate responses to viruses.