A new freshwater red alga is described from Kōke‘e State Park on the island of Kaua‘i, Hawai‘i. The alga is a member of the genus Kumanoa, which was recently established to accommodate two sections of the large freshwater red algal genus Batrachospermum. Specimens are shown to be unique relative to all other confirmed members of the genus based on both morphological comparisons and molecular phylogenetic analyses of a concatenated alignment of three molecular markers. Kumanoa alakaiensis Sherwood, Jones, & Conklin, n. sp., differs from the only other known Hawaiian representative of the genus, K. spermatiophora, which is endemic to Maui, in having a much smaller, cartilaginous thallus; having reduced subspherical- to slightly obconicshaped whorls; and in lacking spermatiophores. Phylogenetic analyses support K. alakaiensis as a distinct and early diverging lineage within the genus, and as sister to a large clade containing all but three species characterized to date. This species description brings the total number of recognized freshwater red algal species in Hawai‘i to eight, three of which are believed to be endemic.