This manuscript deals with the first records of five jellyfish species in the Marmara Sea (Turkey) of previously unrecorded medusozoan species (four hydrozoan taxa, Liriope tetraphylla , Aglaura hemistoma , Solmundella bitentaculata , Neoturris pileata , and one scyphozoan, Paraphyllina ransoni ) from plankton collections made in the years 2001–2008. Jellyfish abundances and distribution patterns are reported here, with a discussion about their possible introduction pathways and potential predatory outcomes into the fragile pelagic ecosystem of the Marmara Sea.