A taxonomic survey of marine and estuarine benthic green algae has been conducted at Lord Howe Island, the site of the world’s southernmost (at 31.5°S) consolidated coral reef. Thirty-two genera and 71 species are described from habitats ranging from upper intertidal to 20–30-m depths. Sixteen of the species are based on Lord Howe types, and 11 are described as new. Twelve species are currently known only from the Island. Particularly well represented in the flora are the genera Enteromorpha (six species, one variety), Chaetomorpha (four species), Cladophora (13 species), Codium (five species), Caulerpa (seven species, two varieties) and Bryopsis (three species). The island is further distinctive in having two representatives of some relatively obscure genera ( Cladophoropsis , Rhipiliopsis , Pseudochlorodesmis ) and in being the highest-latitude collecting locality for several species. Affinities of the Lord Howe flora are primarily tropical Indo-Pacific, with very few elements otherwise restricted to southern Australia and New Zealand.