Polymastia echinus is a species of demosponge belonging to the family Polymastiidae. It is only known from shallow subtidal habitats off Goat Island in the far north of the North Island of New Zealand. This is an encrusting sponge with an extraordinary appearance. The sponge is always largely covered in sand, grit and fragments of shells apart from smooth, cylindrical, peach-coloured papillae up to 15 mm in height which are always free of such material. The specific name refers to this rather hedgehog-like appearance.