Viral infections continue to represent major challenges to public health, and an enhanced mechanistic understanding of the processes that contribute to viral life cycles is necessary for the development of new therapeutic strategies(1). Viperin, a member of the radical S-adenosyl-l-methionine (SAM) superfamily of enzymes, is an interferon-inducible protein implicated in the inhibition of replication of a broad range of RNA and DNA viruses, including dengue virus, West Nile virus, hepatitis C virus, influenza A virus, rabies virus(2) and HIV3,4. Viperin has been suggested to elicit these broad antiviral activities through interactions with a large number of functionally unrelated host and viral proteins(3,4). Here we demonstrate that viperin catalyses the conversion of cytidine triphosphate (CTP) to 3'-deoxy-3', 4'-didehydro-CTP (ddhCTP), a previously undescribed biologically relevant molecule, via a SAM-dependent radical mechanism. We show that mammalian cells expressing viperin and macrophages stimulated with IFN alpha produce substantial quantities of ddhCTP. We also establish that ddhCTP acts as a chain terminator for the RNA-dependent RNA polymerases from multiple members of the Flavivirus genus, and show that ddhCTP directly inhibits replication of Zika virus in vivo. These findings suggest a partially unifying mechanism for the broad antiviral effects of viperin that is based on the intrinsic enzymatic properties of the protein and involves the generation of a naturally occurring replication-chain terminator encoded by mammalian genomes.