Five species of Prionospio Malmgren, 1867, each with four pairs of branchiae, are studied from coast of the Iberian Peninsula. Two of these species, Prionospio cristaventralis sp. n. and P. parapari sp. n., are new to science, whereas P. caspersi Laubier, 1962, P. fallax Söderström, 1920, and P. ehlersi Fauvel, 1928 have been previously recorded. Prionospio cristaventralis sp. n. is characterized by having ventral crests present on chaetigers XI–XIX, dorsal crests and low crests on chaetigers X–XXXIV, triangular neuropodial postchaetal lamellae with pointed ventral edges on chaetiger II, oval neuropodial lamellae on chaetiger III, digitiform pinnules on the posterior face of the first and fourth pairs, and branchial pairs II and III are triangular. Prionospio parapari sp. n. is characterized by having rounded neuropodial postchaetal lamellae on chaetiger I, digitiform pinnules on the posterior face of the first and fourth pairs, branchial pairs II and III are cirriform, low dorsal crests on chaetigers VIII–IX, and oval neuropodial lamellae with enlarged dorsal edges on chaetiger III. A key is given to all Prionospio species with four pairs of branchiae known from the Iberian Peninsula coastline.
