The hoplonemertean Vieitezia luzmurubeae, gen. et sp. nov. is described from specimens collected in a national park on the north-west Iberian Peninsula, the Parque Nacional Marítimo-Terrestre das Illas Atlánticas de Galicia. The species, previously mistaken as the Mediterranean species Tetrastemma vittigerum (Bürger, 1904), is frequently associated with two common species of sea squirt, Phallusia mamillata (Cuvier, 1815) and Ciona intestinalis (Linnaeus, 1767), inside which the nemertean completes its life cycle. Some of the specimens examined were protandrous hermaphrodites. Data on morphology and anatomy are provided with illustrations. Sequences of the nuclear ribosomal gene 18S rRNA and the mitochondrial gene cytochrome c oxidase subunit I were compared with those of other hoplonemertean species and all phylogenetic analyses suggested that Vieitezia is sister to the genus Gononemertes, which parasitises ascidians, within a clade also containing the genera Oerstedia and Nemertellina. In contrast, the morphologically similar genus Tetrastemma appears in a separate clade. This study stresses the need for combining molecular and morphological data when studying nemertean biodiversity.