The hydroid specimens for this study were collected by SCUBA diving from about 15 m deep at the Isl. Marado, Jeju-do, Korea. Among the identified species $Aglaophenia$ $latecarinata$ Allman, 1877 was shown to be new to the Korean fauna. This species belongs to the family Plumulariidae which is the second large hydroid taxon in Korea. The distinct morphological characteristic of this species is the nine marginal teeth of hydrotheca, one median abcauline and four pairs laterals. Median abcauline tooth is bifid, with a broad keel extending along the front of the hydrotheca from its margin to the mesial inferior nematotheca, from which the species name was originated. Up to date four aglaophenian species of 35 plumularians have been reported from Korea.