Widespread data deficiencies for marine invertebrate taxa undercut our ability to understand extinction risks and manage threats. Despite being listed as a threatened species since 1999, the Tasmanian endemic Bruny Island sea star, Smilasterias tasmaniae, has largely been overlooked. The uncertain status of S. tasmaniae populations represents a critical gap in regional and global understandings of the threats facing sea stars. This study assessed the population status of S. tasmaniae, using intertidal field surveys and museum specimens to infer changes through time. We found that S. tasmaniae populations are small, fragmented, and declining in both abundance and extent. The species is now known to occur at only five sites on Bruny Island, Tasmania, with major threats including the development of coastal infrastructure in key rocky intertidal habitat. Smilasterias tasmaniae is part of a larger landscape of regional and global biodiversity loss, acting alongside the Tasmanian endemic Derwent River sea star (Patiriella littoralis, likely extinct) and live-bearing sea star (Parvulastra vivipara, Critically Endangered) as indicator species for an increasingly degraded marine ecosystem. Our findings provide a crucial baseline for local conservation efforts and underscore the urgent need to study and protect marine invertebrates in a global warming hotspot.