A new species of the clionaid genus Pione is described from the south-western Atlantic in Argentine waters. Pione angelae sp. nov. is the first record of the genus from southern South America. It occurs on shells of Olivancillaria urceus and Buccinanops monilifer, two common gastropods living off Mar del Plata, Buenos Aires, Argentina (38°01′41″S 57°31′07″W) that are inhabited by the hermit crab Loxopagurus loxochelis. Pione angelae sp. nov. is visible on the shell surfaces as circular papillar perforations of 312 μm on average with regular diameters. They lead directly into interconnected chambers from both sides of the shell. The chambers form well-defined galleries in dense substratum. Straight tylostyles in two size-classes (defined by their length: width ratio) occur: 158.6 μm length×2.1 μm width and 185.0×4.5 μm, with spherical heads. Microscleres are more common than tylostyles, with microspined acanthoxeas of 91 μm length, slightly bent in the centre, as well as spirasters of 12 μm length with spines more concentrated on the crests of the spirals; rarely they are straight (microrhabd like). The present samples are compared to similar valid species of the same genus and it is concluded that they represent a new species.