An integrative molecular and morphological study is presented for the family Unidentiidae. Molecular phylogenetic analyses were conducted with the inclusion of all previous and newly obtained molecular data for the family Unidentiidae Millen Hermosillo 2012. A new species of the genus Unidentia Millen Hermosillo 2012, U. aliciae sp. nov., is described from Thailand as part of an inventory of sea slugs at Koh Tao. All up-to-date available morphological data for the species of the genus Unidentia is for the first time summarized. Morphological differences among the different species of Unidentia are clarified showing that every species has its own distinguishable morphological traits. According to the new molecular and morphological data, the family Unidentiidae is re-confirmed as a well-supported taxon of the aeolidacean nudibranchs. The taxonomy and phylogeny of the Aeolidacea in the light of the family Unidentiidae is briefly discussed and necessity of a fine-scale and narrowly-defined taxa approach instead of a ''superlumping'' one is highlighted.