A species of the genus Amorphinopsis is described for the first time for the Atlantic Ocean. The new species was described based on the study of 25 specimens, collected in the area of the São Sebastião Channel and its environs (northern sector of São Paulo State coastline) and in the Cabo Frio region (Rio de Janeiro State). The form is massive cushion-shaped, lobate, occasionally encrusting. The megascleres are styles [160–260 (N=20)/5–10 (N=10) μm; length/thickness] and oxeas [150–900 (N=100)/5–18 (N=20) μm]. Amorphinopsis atlantica sp. nov. differs from the other species of Amorphinopsis by its colour, dark-greyish-green with or without yellow tinges on the exposed surface, and the smaller size of its oxeas. Amorphinopsis excavans is the closest species to the Brazilian material, but can still be set apart by a series of smaller traits, such as oxeas and styles never overlapping (the smaller oxea is always larger than the larger style).