Gymnophalloides seoi n. sp. (Digenea: Gymnophallidae) is described from worms expelled from a naturally infected women in Korea. The patient, a 66-yr-old housewife, experienced severe epigastric discomfort, and fecal examination revealed numerous parasite eggs of 0.019-0.021 x 0.014-0.016 mm. After anthelmintic treatment and purgation, 952 digeneans consisting of 910 adult gymnophallids and 42 heterophyids (2 species) were recovered. The gymnophallid flukes closely resemble Gymnophalloides tokiensis Fujita, 1925 (known only as metacercariae from oysters), type and only previously known species of the genus, except for a difference in the position and orientation of the seminal vesicle. Without experimental proof of identity, the adult gymnophallids have been described as a new species, G. seoi, the only gymnophallid known to infect humans.