Two new species of the genus Nephasoma, phylum Sipuncula, are described from subtropical waters of the western Atlantic. Nephasoma (Nephasoma) columbaris is described from the central east coast of Florida where it is found along the edge of the continental shelf at depths of 61 to 104 m associated with the rubble from the reefs of the deep water coral Oculina varicosa. A second species, Nephasoma (Nephasoma) ditadii, is described from Brazilian waters in sand habitats at 150 m on the continental shelf off the coast of the state of São Paulo. Morphology of the two species is compared with allied species in the genus and the phylum. Usage in the taxonomic literature of the diagnostic characters “shield” and “pseudoshield” is reviewed and evaluated.