Cnidarian jellyfish are pest species of marine aquaculture, with their acute impacts well documented in farmed fish, particularly affecting the delicate gill tissue. Less is known about subsequent consequences of exposure, including their association with secondary bacterial infections. The aim of this study was to identify whether potentially pathogenic bacteria were present as part of the adherent microbial populations of three medusozoan jellyfish species; Obelia geniculata, Neoturris pileata and the Lion's Mane jellyfish Cyanea capillata. A number of potential pathogens of commercial aquaculture were identified, including Aeromonas salmonicida, Vibrio splendidus and Vibrio alginolyticus. These findings suggest that jellyfish must be considered not just agents of direct trauma to fish in aquaculture, but also potential reservoirs and vectors of bacterial disease. Since jellyfish may be increasing in the coastal waters where marine aquaculture occurs, the role of jellyfish in bacterial outbreaks has implications for marine aquaculture globally.