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During an investigation of a natural sandy beach on Jeju Island, South Korea, a species of marine nematode belonging to the rare genus Manunema was discovered and is now being reported. Up until now, no molecular sequences of this genus have been available, and its position within the nematode system has been a subject of dispute. Currently, Manunema is classified within the order Plectida, either as the only genus of the family Peresianidae in the superfamily Haliplectoidea or directly as a genus of the family Leptolaimidae. Here, we report on the discovery of Manunema cf. proboscidis Gerlach, 1957, which was found off Jeju Island, South Korea. We also conduct an in-depth investigation of its peculiar morphology, with a particular focus on the reproductive system type observed within the species. Additionally, we examine the phylogenetic relationship of the genus using 18S rRNA sequences. Partial mtCOI, nearly complete sequences of the 18S ribosomal RNA gene as well as a partial 28S (D2–D3 region) gene are provided. Morphologically, the presence of tubular precloacal supplements (shared by superfamily Leptolaimoidea and not Haliplectoidea) and antidromously reflected ovaries of females in Manunema (shared by the order Plectida and not Desmoscolecida) may have taxonomic significance. Our phylogenetic trees based on 18S rRNA also reflected this, including Manunema in a well-supported monophyletic clade of the family Leptolaimidae. [ABSTRACT FROM AUTHOR]