Microbial parasitism, infection, and symbiosis in animals often modulate host endocrine systems, resulting in alterations of phenotypic traits of the host that can have profound effects on the ecology and evolution of both the microorganisms and their hosts. Information about the mechanisms and genetic bases of such modulations by animal parasites is available from studies of steroid hormones. However, reports involving other hormones are scarce. We found that an insect virus, a betaentomopoxvirus, encodes a juvenile hormone acid methyltransferase that can synthesize an important insect hormone, the sesquiterpenoid juvenile hormone. Phylogenetic analysis suggested that this gene is of bacterial origin. Our study challenges the conventional view that functional enzymes in the late phase of the juvenile hormone biosynthesis pathway are almost exclusive to insects or arthropods, and shed light on juvenoid hormone synthesis beyond Eukaryota. This striking example demonstrates that even animal parasites having no metabolic pathways for molecules resembling host hormones can nevertheless influence the synthesis of such hormones, and provides a new context for studying animal parasite strategies in diverse systems such as host-parasite, host-symbiont or host-vector-parasite.