Colonial ascidian, Eusynstyela monotestis (Tokioka, 1953), is newly reported from Korean waters. The specimens of E. monotestis examined in this study were collected at subtidal zone of Beomseom, Munseom, Seopseom and Chagwido in Jeju-do by SCUBA diving. The genus Eusynstyela Michaelsen, 1904 is also new to Korean waters and it is distinct from other genera by having branchial sac with folds, longitudinal stigmata, hermaphrodite gonads on both sides, 1-2 male follicles in each gonad and body wall with endocarps. Eusynstyela monotestis is distinct from other species by having gonad with only single male follicle. In this paper, detailed descriptions and photographs of Eusynstyela monotestis (Tokioka, 1953) are provided.