Freshwater specimens of an encrusting red alga were collected from rivers in Tahiti, French Polynesia. Observed morphological features corresponded to those of the genus Hildenbrandia (Hildenbrandiales, Florideophyceae), confirmed by genetic data. rbcL and 18S rDNA sequences of the Tahitian specimens were 4.95-5.15 % and 0.94 %, respectively, identical to those of a specimen from the Philippines. The Tahitian and Philippine lineages formed a well-supported monophyletic clade, which positioned basally to the rest of the freshwater Hildenbrandia clade. Tahitian specimens cells were 7.6-16.2 μm in diameter in surface view, rectangular in cross-section, 11.6-16.4 μm long and 2-3 μm high, and presented filaments 37-240 μm long, occasionally branching. While the cells were overall wider than in other freshwater Hildenbrandia species, no other outstanding morphological differences were observed between the Tahitian and other species of Hildenbrandia. The genetic divergences and longer cell sizes between the Tahitian and Philippine lineages strongly indicated they were not conspecific. A formal description of the new species, Hildenbrandia tahitiensis sp. nov., is provided. This is the first report of the genus Hildenbrandia from French Polynesia. The present documentation of Hildenbrandia in a South Central Polynesian locality extends our knowledge of the geographical range for this genus.