Adult male Diamysis cymodoceae sp. nov. are unique within the family Mysidae by having a peculiar furry covering on the postero-lateral portions of the carapace. This phytophilic mysid inhabits meso- to metahaline waters in a wide arch from the Adriatic, Ionian, and Aegean seas in the E Mediterranean, up to the Sea of Marmora, and the SW Black Sea. A supplementary description of D. mecznikowi (Czerniavsky, 1882) from the Black Sea underlines differences between this and the new species. Considerable extensions of known ranges are reported for D. mesohalobia heterandra Ariani & Wittmann, 2000, to the Marmora basin, for D. bacescui Wittmann & Ariani, 1998, to the island of Cyprus, and for D. lagunaris Ariani & Wittmann, 2000, to the island of Crete. First records from the Algarve coast suggest a conjunction between the NE Atlantic and the Mediterranean populations of D. lagunaris. This could point to an indigenous status of the Atlantic populations, whereas the occurrence in (semi-)artificial water bodies may (on the contrary) confirm a previously proposed non-indigenous status. The ecological diversification within the genus Diamysis Czerniavsky, 1882, mainly pertains to preferences for different salinity conditions and different plant substrates. An updated key to the species is given.