Climate change driven re-assembly of ecological communities presents a threat to biodiversity. As oceans warm, pelagic larvae can disperse further within their thermal niches, facilitating new biotic interactions. Understanding species thermal niches is critical in understanding how ecological communities may re-assemble in the future. The echinoid genus Tripneustes is an ecologically and commercially important tropical and temperate taxon. In eastern Australia, two species of Tripneustes occur, the tropical T. gratilla gratilla, and the subtropical/temperate T. australiae. Until recent taxonomic clarification, the presence of T. australiae in temperate Australia was misidentified as a climate change driven range extension of T. g. gratilla, presenting a knowledge gap in our understanding of the biology and ecology of these species. We aimed to construct the true distributions of T. g. gratilla and T. australiae in eastern Australia. We did so to understand the ranges of these species and construct their realized thermal niches, to predict how their distributions may change with ocean warming. To do so we used natural history collections (Australian Museum, Museums Victoria, Natural History Museum London, The Sea Urchin Science Centre and Gallery) and citizen science initiatives (Reef Life Survey) to maximise the area of observation. We found a clear tropical distribution of T. g. gratilla with a broad latitudinal range from Torres Strait to Jervis Bay (9° 56' 2.4" S ®35° 7' 12" S) and a narrower subtropical/temperate distribution of T. australiae from Byron Bay to Narooma (28° 37' 0.12" S ® 36° 15' 0" S). The tropical T. g. gratilla has a realized thermal niche from 28 – 18.4°C, while T. australiae has a narrower realized thermal niche from 24.3 – 17.8 °C. Densities of Tripneustes spp. are the highest in the subtropical Tweed-Moreton and Lord Howe and Norfolk Island ecoregions of Australia where both species co-occur.