Rosacea flaccida, a new prayine siphonophore, is described from specimens collected by SCUBA divers in the upper 30m of the subtropical and temperate North Atlantic Ocean. The new species has stoutly cylindrical, flaccid nectophores and delicate flattened bracts. The nectophores are morphologically similar to those of R. plicata sensu Bigelow, 1911 and R. cymbiformis (delle Chiaje, 1822) having a deep hydroecial groove and meandering lateral radial canals in the nectosac. In one of the nectophores there is a slight dorsal prolongation of the somatocyst at its apical end into the mesoglea. The eudoxid bracts are distinctive, being flattened dorsoventrally and divided, on the proximal side of the stem, into two lobes which are twisted at an angle of approximately 90° to the lobe on the distal side. Right and left longitudinal bracteal canals are well developed. The origin of the dorsal bracteal canal from the right longitudinal canal differs from that in the other Rosacea species but resembles the configuration found in the bracts of species of the genus Praya.