This is the first contribution to a worldwide taxonomic revision of the closely related genera Haplosyllis (the main goal is to describe species within the Haplosyllis spongicola complex) and Geminosyllis. The type species, Haplosyllis spongicola, is re-described based on the syntypes and other material collected from Spanish seas. A combined taxonomic and morphometric (i.e. PCA and discriminant analysis) approach was used to study species of the Haplosyllis spongicola complex from Spanish seas. Two of them are described here as new: Haplosyllis carmenbritoae n. sp. from the Canary Islands, and Geminosyllis granulosa n. sp. from Iberian and western Mediterranean waters, and which is attributed to Geminosyllis due to the presence of a pharyngeal trepan. The third species, which also belongs to Geminosyllis, is morphologically very close to, and morphometrically indistinguishable from, H. spongicola, except for the presence of the trepan. This casts serious doubts on the relevance of this structure for separating the two genera and thus prevents us from describing this last species as new.