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The distribution of heterobranch sea slugs is generally poorly documented at a regional scale. Thus, it is currently difficult to quantify biodiversity, identify endemic and invasive species, and track range shifts at scales relevant to conservation management. For Lord Howe Island, which lies ~600 km east of the New South Wales (NSW) mid-north coast, data from a range of taxa indicate high biodiversity and endemism, but this has not been examined for heterobranch sea slugs. To address this deficit, we collated occurrence data on sea slugs from both private and public sources, including museum records, scientific literature, field guides and citizen science activities. A total of 186 nominal (formally described) species in 82 genera and 31 families were identified from intertidal and subtidal habitats. Of these, two species are endemic to Lord Howe Island, two have not been recorded elsewhere in Australia, and 28 have not been recorded on the mainland coast of NSW. These results support studies of other taxa suggesting that the relative isolation of the island has facilitated the development of diverse and unique assemblages. However, this isolation is moderated by larval transport from surrounding regions, resulting in considerable overlap of the species pool with the mainland coast of NSW and tropical areas to the north.