How antiviral immunity first arose in animals is a central question in evolutionary biology. Using the sea anemone Nematostella vectensis, we identify CARDIB, a previously uncharacterized gene located next to RLRb, a cnidarian homolog of the vertebrate RIG-I-like receptor family. This conserved genomic linkage across Anthozoa reveals an ancient coupling between immune sensing and regulation. Despite sequence similarity to vertebrate MAVS, CARDIB performs an opposing function: it represses immune