Castroviejoa Galbany, L. Sáez & Benedí has been recently described as a new genus of Gnaphalieae, and comprises the two Mediterranean species Castroviejoa frigida (Labill.) Galbany, L. Sáez & Benedí and C. montelinasana (Schmid) Galbany, L. Sáez & Benedí, formerly known as Helichrysum frigidum (Labill.) Willd. and H. montelinasanum Schmid. In the present paper their morphology is accurately described and compared with that of Helichrysum and related genera. These two species present three morphological characters that are not found together in other Helichrysum species, and which characterise the genus Castroviejoa: the multicellular, biseriate, glandular hairs present on the corolla tube; the undivided stereome; and the apical sweeping hairs that also extend dorsally along the style branches. The full taxonomic treatment of the new genus is provided.