A new species of Enchytraeidae, Marionina triplex sp. n. is described from an intertidal mussel bank in Sinop, southern Black Sea coast of Turkey. Marionina Michaelsen, 1889 is a widely distributed and heterogeneous assemblage of species, but while awaiting a revision the new species is placed here, as it appears to belong to a group of small enchytraeids currently associated with this genus. The new species in some aspects resembles other species reported from the Mediterranean-Black Sea area [M. adriaticus Vejdovsky, 1877; M. argentea (Michaelsen, 1889); M. brevis Finogenova, 1972; M. spicula (Frey & Leuckhart, 1847)] and some species described from elsewhere (M. cana Marcus, 1965; M. singula Ude, 1896,- M. sublitoralis Erseus, 1976; M. ulstrupae Healy, 1996), but it can be distinguished by its body length, number of segments, number and form of chaetae, shape and size of pharyngeal glands and spermathecae, etc. The new species is only 1.4-1.8 mm long, with 20-25 segments, and is thus one of the smallest marine enchytraeids known. It has mostly three chaetae per bundle, and spermathecae with the ampulla of about the same size as the ectal duct, and with a few gland cells attached to the base of the ectal duct.