Rhodachlya hawaiiana Kurihara, J.A. West, Conklin et A.R. Sherwood sp. nov. is described based on cultured material from the island of Hawaii as the second member of the genus Rhodachlya. The pit plug ultrastructure of this alga is characterized by the presence of inner and outer cap layers but lacking a cap membrane between the layers, which is a diagnostic feature of the order Rhodachlyales and the genus Rhodachlya. Morphologically, R. hawaiiana is distinctive from R. madagascarensis in vegetative cell dimensions, frequency of colorless hair cell formation, and spore germination pattern. Rhodachlya hawaiiana and R. madagascarensis are also distinguishable by their nuclear SSU rRNA gene sequence divergence (p-distance = 0.0064), which is comparable to other species-level divergences among members of the Nemaliophycidae.