Crustose corallines were collected from a wide range of depths (intertidal to about 300 m) throughout the Hawaiian Archipelago. A total of 25 species in 10 genera are recognized on the basis of habit, anatomy, morphology, and ecology, including one new genus and 10 new species. Generic and specific keys for the differentiation of the Hawaiian crustose corallines are also provided.The ecology of each species, in terms of depth distribution and habitat, is also given, and the potential use of these plants in determining paleoenvironments in the Hawaiian Neogene is discussed.The Caribbean and Hawaiian crustose coralline floras are briefly compared. The large number of “pair species” and the parallelism in subfamily, generic, and “pair species” ecology indicate that coralline evolution is very slow. The crustose corallines are potentially excellent paleoecological indicators for the Tertiary.