Callitriche is a genus of about 75 species of small herbs, ranging from aquatic to amphibious and terrestrial, with a complicated and incompletely resolved taxonomy. This work contributes to the taxonomic knowledge of the genus Callitriche in Oceania and Australasia by presenting descriptions and maps, together with information on habitats and the conservation status of 21 species, including taxonomic novelties, as well as revision of some critical species. Callitriche fuliginea and C. insularis are described as new species from New Guinea and French Polynesia, respectively, C. chathamensis and C. petriei are recognised as separate species, C. capricorni and C. cyclocarpa are reinstated.