Cone snails are biomedically important sources of peptide drugs, but it is not known whether snail-associated bacteria affect venom chemistry. To begin to answer this question, we performed 16S rRNA gene amplicon sequencing of eight cone snail species, comparing their microbiomes with each other and with those from a variety of other marine invertebrates. We show that the cone snail microbiome is distinct from those in other marine invertebrates and conserved in specimens from around the world, including the Philippines, Guam, California, and Florida. We found that all venom ducts examined contain diverse 16S rRNA gene sequences bearing closest similarity to bacteria. These sequences represent specific symbionts that live in the lumen of the venom duct, where bioactive venom peptides are synthesized. In animals, symbiotic bacteria contribute critically to metabolism. Cone snails are renowned for the production of venoms that are used as medicines and as probes for biological study. In principle, symbiotic bacterial metabolism could either degrade or synthesize active venom components, and previous publications show that bacteria do indeed contribute small molecules to some venoms. Therefore, understanding symbiosis in cone snails will contribute to further drug discovery efforts. Here, we describe an unexpected, specific symbiosis between bacteria and cone snails from around the world.