Diseases of sea urchins have been implicated in dramatic transitions of marine ecosystems. Although no definitive causal agent has been found for many of these outbreaks, most are hypothesised to be waterborne and bacterial. Here we show the first report of a novel disease affecting at least 2 species of urchins off the south-eastern coast of Australia. The aetiological agent, identified via a range of molecular techniques, immuno-histology and inoculation experiments, was found to be the opportunistic pathogen Vibrio anguillarum. The disease appears to be temperature-dependent, with a faster transmission rate and increase in prevalence during ex - perimental trials conducted at higher temperatures. Furthermore, analysis of long-term field data suggests that it may have already reached epidemic proportions. With the increases in ocean temperatures brought about by climate change, this novel urchin disease may pose a severe problem for the organisms associated with the temperate reefs off Australia and/or the ecosystem as a whole.