Cosmioneis Mann et Stickle is a pennate diatom in the family Cosmiondeidaceae and order Naviculales. A single species of this genus (Navicula pusilla W. Sm., now Cosmioneis pusilla (W. Sm.) Mann et Stickle), had been previously reported from the island of Oahu, in the Hawai'ian archipelago. In extensive surveys of algae of subaerial habitats in Hawai'i we collected samples from the islands of Maui and Oahu, from which we distinguished populations of three species of Cosmioneis. Reports from other habitats describe C. pusilla as morphologically variable with valve ends ranging from subrostrate to strongly capitate. Our populations of Cosmioneis showed little variability and were either strongly capitate, rostrate or broadly lanceolate. We discuss the morphology and ecology of Cosmioneis in Hawai'i and propose three new species in this genus, Cosmioneis reimeri, Cosmioneis citriformis and Cosmioneis hawaiiensis. Light and scanning electron micrographs of the new species are provided.

New taxa: Cosmioneis reimeri Lowe and Sherwood, Cosmioneis citriformis Sherwood and Lowe, Cosmioneis hawaiiensis Lowe and Sherwood