During a systematic study on Korean nudibranchs, two phyllidiid and two polycerid species were collected and identified with examination of their external morphological characters. As a result of the study, two phyllidiid species, Phyllidia ocellata Cuvier, 1804 and Phyllidiella pustulosa (Cuvier, 1804) were redescribed since there was no descriptions of these species with Korean specimens. And two polycerid species, Thecacera pennigera (Montagu, 1815) and Triopha catalinae (Cooper, 1863) were described as new to Korean fauna. Diagnostic characters of these species are described with illustrations in this study. DNA barcode data from three species examined were also provided for a supplement to morphological identifications.