Published behavioural, DNA sequence, and morphologic data on six populations of Mastigias, five from marine lakes and one from the lagoon, in Palau are summarized. Each marine lake population is distinguished from the lagoon population and from other lake populations by an unique suite of characteristics. Morphological differences among Mastigias populations in Palau are greater than those recorded between any previously described species of Mastigias, whereas molecular differences are far less (≤2.2% in COI and ITS1) than those found between medusae identified in the field as Mastigias papua (>6% in COI and ITS1). Thus, each marine lake population in Palau is described as a new subspecies of Mastigias cf. papua: remengesaui (in Uet era Ongael), nakamurai (in Goby Lake), etpisoni (in Ongeim'l Tketau), saliii (in Clear Lake), and remeliiki (in Uet era Ngermeuangel).